So I was able to get HD to configure down to a biscotti build and it fits (barely, but it's 1022 bytes, just like the real biscotti which is a little surprising).
So I went ahead and made a combinatoric makefile that can be given firmware config files, grouped by the minimum chip they can run on, and then it builds all valid combinations of hex files.
I think can say I've fully re-united the bistro family code base. I have not put in modegroup.h files for the original bistro mode definitions (done) or the original trippledown modegroups yet(done), but it's simple to do (done) and along with a configuration to match the original bistro build (done).
The Makefile works directly from Atmel studio and can be used by selecting it from the Project->Properties->Build pane. Of course, I'll have to post it up first.
At the end the makefile presently spits out this:
text data bss dec hex filename
Live child 01298C28 (sizes) PID 19451072
text data bss dec hex filename
1022 0 34 1056 420 bistro-biscotti-attiny13.elf
1036 0 34 1070 42e bistro-biscotti-attiny25.elf
1040 0 34 1074 432 bistro-biscotti-attiny45.elf
1040 0 34 1074 432 bistro-biscotti-attiny85.elf
410 0 37 447 1bf bistro-battcheck-divider-attiny13.elf
426 0 37 463 1cf bistro-battcheck-divider-attiny25.elf
430 0 37 467 1d3 bistro-battcheck-divider-attiny45.elf
430 0 37 467 1d3 bistro-battcheck-divider-attiny85.elf
1748 0 42 1790 6fe bistro-bistro-HD-attiny25.elf
1752 0 42 1794 702 bistro-bistro-HD-attiny45.elf
1752 0 42 1794 702 bistro-bistro-HD-attiny85.elf
1732 0 41 1773 6ed bistro-TAv1OTC-attiny25.elf
1736 0 41 1777 6f1 bistro-TAv1OTC-attiny45.elf
1736 0 41 1777 6f1 bistro-TAv1OTC-attiny85.elf
1158 0 38 1196 4ac bistro-BLFA6_EMU-attiny25.elf
1162 0 38 1200 4b0 bistro-BLFA6_EMU-attiny45.elf
1162 0 38 1200 4b0 bistro-BLFA6_EMU-attiny85.elf
1696 0 38 1734 6c6 bistro-classic-attiny25.elf
1700 0 38 1738 6ca bistro-classic-attiny45.elf
1700 0 38 1738 6ca bistro-classic-attiny85.elf
1658 0 37 1695 69f bistro-trippledown-attiny25.elf
1662 0 37 1699 6a3 bistro-trippledown-attiny45.elf
1662 0 37 1699 6a3 bistro-trippledown-attiny85.elf
454 0 38 492 1ec bistro-battcheck-Vcc-attiny25.elf
458 0 38 496 1f0 bistro-battcheck-Vcc-attiny45.elf
458 0 38 496 1f0 bistro-battcheck-Vcc-attiny85.elf
So it's turning into a firmware factory. I'm not sure if there's actually any difference between the 45 and 85 versions. I guess I could diff them and see. There's no difference in the code (there is for the 25 though).
Updated: added battcheck builds to the list.