Atlantis said:
If you use sup subtitles which are for bluray, they will be images, so you can make them any way you want.
The fact that they are for BD doesn't mean they'll work well with AVCHD.
As for them to slightly move and have grain, I think it's way overkill and not necessary. They really didn't intend them to be moving originally!
They didn't originally mean to have matte-lines in composites either, yet it was a limitation of the technology at the time and if it was possible, I'd love to see every last matte-line that was originally there.
If you don't burn in the subtitles, your release will be international and could be spread more. Other countries can have their own subtitles there.
Other countries can have their own subtitles even if there are burned in subs. And those subs would be there if this was a scan of a print - and they would be grainy and shaky.