I don't think you could find such a mapper, as I'm pretty sure ROM chips (which the mappers were originally designed to interact with) don't come in non-power-of-2 sizes. 
(yeah, there's some ROMs with sizes like 160KB, 192KB, 384KB. But that's because they used two ROMS of different sizes, with one holding the graphics data, and the other holding everything else.)

Expanding a 256KB MMC1 ROM to 512KB is possible, but it's also a unique and more difficult expansion.

There's already a hack ideas thread.

...somewhere around here.

Ok, it looks like MMC3 is the way to go. Mirroring won't be an issue. Looks like it'd do what I need.

I figured the non-power-of-two thing was a long shot. I know that ROM chips come in power-of-two sizes, I just thought there might be some mapper that supported an odd number of smaller chips. Oh well, not a big deal. 512K isn't very large by today's standards. (Besides, zero filled banks should compress well, so it's proabably irrelevant.)

Thanks again.

A while back, there was a thread about hacking SMB2 (not J) back into Doki Doki Panic. One of the respondants to that thread said it'd proabably be easier to port Doki Doki Panic from the FDS to the NES, and procceed to do some work on doing so. It didn't sound like it was actually that hard to do. (Of course, the poster (I wish I could remember who it was) was an experianced ROM hacker.)

The same sort of techniques could be used to port SMB2J to the NES, which should make it Game Genie compatible. You may want to look into that approch. If you do, MMC3 would proably be a good mapper to use. (Although other mappers may be more suited to the FDS's program structure, I don't know that much about the FDS). It's a rather nice mapper. It's easy to program, has good flexibilty, but isn't loaded with extra features.

Yeah that was me.  I ended up eating my boot on that attempt, though.  Turns out DDP is loaded with a bunch of self-modifying code that made it really hard to convert.  It also loaded its files on odd boundaries (making converting to bankswapping kind of tricky).

At any rate -- a FDS->NES conversion of DDP would probably be easier than a SMB2->DPP conversion in the sense that there are a lot of really small things that were changed between the games -- and if you wanted a "complete" conversion, a lot of stuff would need to be recoded from scratch.  But the FDS->NES would be harder in the sense that it would take a lot of code disassembling and reworking.

In the end I gave up on my conversion attempt simply because it required more work than I was willing to put into it.  Once I found all the self-modifying code I just said "fuck it -- this isn't worth my time -- I'll just play the FDS version" and dropped the project.  I don't even know if I still have my files/notes anywhere... my computer is kind of a mess  =x


And of course one of the big hurdles with the FDS->NES convert that I didn't even get to was the sound.  DDP uses the FDS channel for sound effects and some music.  In SMB2, the sound effects were switched to the DMC, but the music was rescored to not use the FDS channel.