To compile under linux I just brutally ripped out all the arduino-specific code (I2C etc.), changed a few things like Serial.print to printf, strcpy_p to strcpy, etc. plus had to move a few declarations around. There are still a ton of compiler warning messages that I'll try to look at, and the resulting binary exits with stack overflow as soon as I type anything to it. So I'll try to fix it, and maybe parametrize the types to use specific word sizes like int16_t instead of int, etc. I know lots of ARM boards support the arduino api now, so it would be nice to have a version that works on both avr and arm.
The 16-bit MSP430 is another interesting possible target because of the cheap Launchpad boards, but it doesn't implement the Arduino api that I know of.
Added: it looks like there are very cheap Arduino Mega (2560) clones available on places like Ali Express, making the Teensy++ somewhat less attractive unless the small size is needed.