Is the source code for the prealphas available?
of course, the link is in the first post of this thread. A list of additional addresses is attached to this
I need to shoot aerial RAW with precise GPS timestamp (well, to the extent possible).
There is GPS support in 5 CHDK ports (including the s100), you can use that as starting point.
Currently I use S100, but SX280 should work better in low-light fast-motion situations
I don't know... the s100 has larger sensor, faster lens.
Also, ideally I'd like to be able to control camera and get photos back via USB w/o writing to the SD card (it will be buffered by the on-board computer and lazily written to its flash)
PTP support is working (except for live view). Direct JPEG download should be possible after implementing filewritetask support (the support code in DryOS r52 ports can be of help - note that that remote shooting has issues
on several cameras).
May I join the porting effort?
Sure. Note that, for the time being, this "fork" is stuck at CHDK trunk rev. 3449.