Hi all, just for the sake of anyone interested doing a search, I had to compile Optiboot tonight for a 1284P running with a 12MHz crystal I just reworked onto my board. Although the 1284P @ 5v, 16MHzhas been stable for me and everyone else I've heard from, it does have issues if there's a voltage sag long before hitting the brownout voltage. The datasheet does show however that the 1284P is stable at 12MHz.
Everything worked great the first time - no problem with tons of serial data at 115200, or delay(), etc. Although I guess there's a few libraries out there that may need to be tweaked for 12MHz.
Anyhow, it was very easy to do as long as you've compiled bootloaders before. This was my first time compiling Optiboot, so I just had to read through the makefile a little bit, add in some lines for a 12MHz version, and of course add in a 12MHz version in boards.txt. If you want a 12MHz bootloader for the 1284P, PM me.