Merge .ass-subtitles into movie
Sorry this is a bit late but I just found your question while doing research on this topic.

If you have access to After Effects, I've developed a script that can import .srt subtitle files (I think you can export .srt from Aegisub). You can download a trial at http://aescripts.com/pt_importsubtitles/

It preserves the timings but not the font or position, although these are easy enough to set in AE. Once imported you can drag the composition straight into Premiere Pro using Dynamic Link for compositing onto your movie.

I've also developed a script that can import .ass files into AE. It's primarily meant for subtitles with karaoke timings, but it should be able to import plain subtitles too.
http://aescripts.com/pt_ssakaraokeanimator/