DVD-BOY said:
Last post here suggests a method to convert srt to DoStudio's DOST subtitle format.
http://netblender.ning.com/forum/topics/bdn-xml-dost-format-subtitle-converter-exe
If you have the option with DoStudio 4, go with HDMV menus rather than BD-J as the discs run far better in older Profile 1.0 Blu-ray players.
I've already converted them; DOST is pretty much BDN+XML with a differently formatted XML file, and I was able to convert ProjectThreepio's 720 SUP files to BDN+XML with no trouble because the .pngs were already there.
As for authoring in HDMV mode, I've only got DoStudio Indie, which is BD-J only. I could upgrade to DoStudio EX for HDMV mode if you guys want to pitch in $2000 :P