I see that we have followed very much the same thinking process.
There is no simple way to have full compatibility without having to replace the LiquidCrystal library as such.
Adding defs to the Library will simply end up in a compiler/linker problem where symbols are multiply defined. This is how the Arduino compilation environment is. On a different environment it would be simpler in the sense that you would point your include and linker libraries to different locations.
In any case, what I am going to do is to have it a replacement library since as far as the 4bit and 8bit control are identical.
Regarding the arduino SDK version, it is just a question of time that all users will migrate to the new version. I don't like messing with compilation flags to make things compatible, but I can understand the need for it. So for the first versions of the library I will use conditional compilation flags to make the library compliant with the 22 version and the 1.0.
I was also thinking about doing a port of the SPI control library but at this point I think the package can go as is.
Thanks for your feedback it has been very useful.