The Teensyduino libraries for all the peripherals on the K20 chip - were, I believe, all done by Paul with a bit of contributed code. Like Arduino's libraries, the goal is to have library code (akin to a #include). And then some, e.g., better/faster SPI to exploit the FIFOs in the K20 chip, better ethernet interface to the WizNet 5xxx chips, etc. The inventory is very impressive. I believe the work is done to get the basics all supported, then improve drivers for the popular apps, like the serial data driven LED arrays (thousand of LEDs), and DMA for the A/D and D/A for fancy sound things, and the ethernet chip.
EDIT: -- added
A large portion of the libraries are ARM hardware equivalents to those done for Teensy2 which is an AVR chip as are Arduinos per se. Thus, a lot of application code for Arduino and most all Teensy 2 code runs with few changes on the Teensy 3, due mostly to the work done in the libraries. SPI is an example... where Paul has extra code for the Teensy 3 to emulate the AVR SPI for the sake of code portability.
Could ARMbasic just wrap the Teensyduino libraries -as a hardware abstraction layer - rather than coding all new I/O run time?