The following information is directly from Rich Testardi - firstname.lastname@example.org - regarding StickOS:
I have made all of the source except the "core BASIC" public in the skeleton source code project available here: http://www.cpustick.com/downloads.htm
From this you can rebuild StickOS and make basic modifications to it. This includes all of the "platform-dependent" code that is involved in porting from MCU to MCU, and obviously all of the code that controls the actual MCU hardware, flash memory, I/O, etc., so all that code can be debugged as well. Folks are free to use/modify/whatever all that source completely without restriction and you can use it to instantly get a C project up and running on any of the supported StickOS MCUs. In fact, the goal is that after you have a BASIC program running, you can trivially port it to C and use all of the exact same advanced functionality available in StickOS (like pin variables, ZigFlea, timers, etc.) from C exactly as you do so in BASIC -- just with better performance. You can also add new BASIC commands, statements, pin functions, etc.
Only the "core BASIC" is in Library form so I can keep it consistent between all platforms -- this consists of 5 source files. A number of people have access to that source code (so you can think of it is limited source access for collaborators), but it's basically just for folks who are helping with development, so we can keep it moving in the right direction. The primary problem is that it has to remain compatible with a dozen different MCUs with different resource abilities, down to very small 8-bit processors, and we have to test the changes everywhere -- I don't want to deal with massively diverging versions. If we have folks who are genuinely interested in helping development, and who want to help support all (or even new!) processors, we can certainly expand this group of folks.
As far as copyright/license goes, I maintain the copyright of the core BASIC code (even open source code always maintains its copyright). Everything else is completely free for educational purposes. For commercial purposes, what I have always told folks is that we eventually expected there might be a $1 or $2 license per MCU just to cover support costs, but that would only happen if the volumes were in the thousands. Since I never expect that to happen, it is essentially license free -- we're not trying to make money, only cover support costs.