Currently, no.  I say currently, because I will be forcing it to do just that in the not too distant future.  Mind you, as you can tell from this extremely outdate picture, there are other ways to add dialogue:


As for your other questions, you'd probably be best to take a gander at:
http://jathys.zophar.net/supermetroid/kejardon/

As for hiring someone to do ASM... Unless you've got some huge undertakings, it'd probably be worth it to learn a bit yourself or find out if anyone would be interested in helping out for free (which some people may do if they see progress in your game and decide that it's good enough that they want to put the effort in).

This is usually not well-received within this community. People really help each other out if they feel like it or if they see that the project is serious but just lacks this tiny edit etc. But taking money for this is generally discouraged and we don't like it around here. It's basically one step away from asking if somebody made you the hack for money when you explain your concept to them...

Other than that, there are some talented Super Metroid hackers around here, some of them might tell you more about that. Also, "hard" limits are usually "hard to do" limits. Everything else is a hardware limitation and those can only very cleverly seemingly be overcome. With enough patience, endurance and masochism, you can do anything you like with that rom.

cYa,

Tauwasser

Not really. There are often multiple ways of doing things, and some things are better to do with ASM, other things are better to do without. But it's uncommon enough that you don't really have to worry about doing something one way and causing a serious roadblock later. Might end up being a headache later, but generally not a serious roadblock.
Data and code is put in the ROM wherever it fits, for the most part. The SNES is very free with mapping stuff, there's only very minor limitations with LoRom (which Super Metroid uses) in banks C0-FF, but the only stuff in C0-FF is compressed room data or music data, neither of which you'd have to worry about since the programming for those is complete and there's not really any need to change the programming. There's also plenty of free space in banks 80-BF so you can probably fit anything you need there.
Free space is at the ends of banks, and is easily noticed because it's a LOT of FF's in a row.

Nope. Whichever works best for you creatively and practically.

That used a quirk of the scrolling to work, and was fairly limited in what it could actually do. It was done by teleporting Samus to a different screen when she crossed a certain line, and had to teleport Samus a multiple of 2 screens horizontally and a multiple of 1 screen vertically, and the destination screen had to be essentially the same as the source screen (changes in the tiles would not be visually updated until they were scrolled off screen and back on). Drew designed the room around that limitation (it's easy to notice if you're looking for it).
If you don't mind using doors though, you can achieve the same sort of effect VERY easily and with no significant limitations. Super Metroid doesn't find rooms from Samus's position on the map; instead, each room contains pointers to the connected rooms, which can be anywhere on the map.

As Jathys said, it's possible although there's no easy way built in at the moment.
Depending on how far you want to take it there's not really a limit on what you can do though. You could make a whole logbook if you really want. Of course, that's a LOT of work.

Easily. It's actually done entirely inside of the block data. What you'd want to do is probably have the block type/BTS create a PLM on projectile collision, have the PLM setup check the collision type and delete itself if it wasn't a plasma beam, and have the PLM animation break the block.
You probably won't understand almost anything I said there, though. You'll need to study both assembly and how Super Metroid works.

I actually have a document on this, I think!
X-ray scope is horribly buggy though and I'm honestly not even going to deal with it personally. But you can control what blocktypes appear as when you x-ray them. Fitting a message into blocks though... I don't think you'd be able to really pull that off very easily. You'd only be able to make blocks appear as other blocks when you x-ray over them, and so you'd have to fit the graphics for the text into the block graphics. Or do something entirely different with assembly. If you want hidden messages you'll probably want to look into programming message boxes.

Editing the size is really trivial. You just change the size and change one byte in the internal header (The header is located at 80:FFC0... I don't remember what byte it is you'd have to change though). For Super Metroid the size of the ROM is only a concern if you're adding a lot of rooms to the game, at which point you'll probably be more concerned about fitting stuff like rooms' enemy population where it's supposed to go.

Haha... this is actually the second time I've had an offer to do Super Metroid assembly for money. XD I have hardly enough free time for myself to do commissions, even if I decided to actually program for money.

Palette swap. There is one... hack, for lack of a better term and in the literal programming sense, that actually chooses a different sprite when Samus is facing forward if she has either suit equipped. But really it's just a palette swap.

It wouldn't be too hard, I don't think. You'd have to duplicate some tables, and then change the parts you want to be different, and add appropriate graphics. It would definitely be a good bit of work though.

Ummmm. In general, yes. As I mentioned, it's possible, but it would be a good bit of work. Definitely not the kind of hack you'd want to try when you're learning assembly.