I have just finished and (more or less) tested pattern copying and clearing and saving to/loading from the EEPROM. It all works, and so does my fairly crude interface. (B+U goes to parameter screens, does not stop playback. B+D goes to load/save page, does stop playback.)
Next up, a couple of variations on looping, probably loops of 1, 2, 4, and 8 patterns, starting at arbitrary patterns (but a loop of 8 will start a 1, and a loop of 4 can't start at anything higher than 5 and so on). This gets tougher, since it will mean a) dissociating what's displayed with what's playing (or making it hard to edit) and b) trying to display more info on the top 8 auxLEDs than you can really easily display on simple LEDs. I'll probably have to resort to blinking, which is kind of inelegant. Unfortunately I'm using all 64 pixels for the pattern display so I can't mess with that part of the screen. The good part is that I made good decisions toward the beginning of writing this thing, so most of this won't be too difficult.
When that's done, I'll post the code for MeggySeq and the listener (that, at least, won't change much if at all) and write up some kind of How-To with pictures and details on the serial cable between the Meggy and the Arduino/Waveshield. Also, I'll list all the things that are wrong with how this thing works and see if somebody else feels like fixing them... 8) (Number one: What I call "BPM" isn't quite the same as the actual beats per minute that play.)
I'll also make sure my code is readable and that there are comments and things pointing out where, if you wanted to make it send MIDI output, you'd do so. I may write up that part myself. even.
I'm also at 8296 bytes used out of about 12k usable on the ATmega chip (after the arduino bootloader and stuff is installed). I may be able to ditch some of what I'm doing with progmem-- if nothing else, removing the #define DEBUG flag will get rid of a bunch of static strings.
I'm really happy with how this is turning out. More photos and detailed movies this weekend.