Another update: I have write emulation kind of mostly working! It turns out that a 2011 SD card has a lot of trouble matching the write speed of a 1984 floppy disk, at least with the type of access patterns seen during floppy emulation!
I had to use a "high speed" (class 10) SDHC card in order to get write emulation to work reliably. Even then, it only works in the case of what I call normal writes, which is what you're doing when copying files in the Finder, or saving documents. This kind of write is actually a pattern of read-write-read-write to the floppy, because the Mac has to look for the sector it wants to update, before it writes the new data. The alternative is what I call continuous writes, where the Mac just blasts out an entire track's worth of data in one pass. Continuous writes are performed when a floppy is initialized, and when you do a full-disk write using a disk copy program, and maybe in some other cases I don't know about. The emulator isn't fast enough to do full disk writes, even with the class 10 SD card.
If I were smarter, I could probably figure out some fancy buffering or something to make continuous writes work, or normal writes with slower SD cards, but I'm tired and my head hurts.
The whole question of write support has mushroomed into a much bigger challenge than I ever imagined, and I'm getting eager to actually build the hardware instead of running a million more tests of various buffering and updating schemes. 
More boring details and commentary at http://www.bigmessowires.com/2011/12/07/floppy-vs-sd-card-write-speeds/
I had to use a "high speed" (class 10) SDHC card in order to get write emulation to work reliably. Even then, it only works in the case of what I call normal writes, which is what you're doing when copying files in the Finder, or saving documents. This kind of write is actually a pattern of read-write-read-write to the floppy, because the Mac has to look for the sector it wants to update, before it writes the new data. The alternative is what I call continuous writes, where the Mac just blasts out an entire track's worth of data in one pass. Continuous writes are performed when a floppy is initialized, and when you do a full-disk write using a disk copy program, and maybe in some other cases I don't know about. The emulator isn't fast enough to do full disk writes, even with the class 10 SD card.
If I were smarter, I could probably figure out some fancy buffering or something to make continuous writes work, or normal writes with slower SD cards, but I'm tired and my head hurts.
The whole question of write support has mushroomed into a much bigger challenge than I ever imagined, and I'm getting eager to actually build the hardware instead of running a million more tests of various buffering and updating schemes. More boring details and commentary at http://www.bigmessowires.com/2011/12/07/floppy-vs-sd-card-write-speeds/