No official specs, only
source code. The above memo is unofficial, written by guessing. But ASS2 samples already work with MPC. You can even mux it into MKV (but not into DSM), altho ASS2 in MKV is working just accidentally, without official support (mkvinfo thinks it is A_TEXT/SSA not /ASS). So... I think basically I'm guessing right.
Apparently Gabest added SSA ver.6 (ASS2) when MPC got a new checkbox "Position subtitles relative to the video frame" around November 2005. At the same time, one new tag {\kt} was introduced, which I didn't know about either.
ASS2 specs are not official, i.e. not yet finalized, so we could say ASS2 is still under construction, hence wish list...
I would say "millisec timing resolution" (a small wish isnt it?) or, more ideally, timing by frame number (maybe unrealistic).
ASS2 is not ASS with new tags, but a new file format. So far there are no new tags that only work in ASS2.
The difference between ASS and ASS2 is relatively small, but not backword compatible. Among other things, the sub text to be shown in a "Dialogue" line is after the 10th comma in ASS2, and after the 9th comma in SSA/ASS.