They could have asked Obsidian to do some in-house VA, by going through Avellone. I'm sure they could have got something.
Avellone is certainly aware of the TSLRCM.
He's also mentioned in a few interviews as well.
I think the professional voicing that is missed the most isn't for the M4-78 EP but for the missing lines at the Trayus Core during the end game. Kreia was supposed to have more lines to comment on the Exile's party but they were never recorded. However, those lines do exist as voiced by Atris' voice actress - a remnant for when meeting Atris at the Core was still something planned for the game. Some of those extra lines can be found
here.
Now if Avellone wants to hire all that expensive talent for us to add party member lines, I'm sure that the modders wouldn't complain! LOL
Interesting fact: one of those droids
is voiced by an actual Obsidian employee. (No, it's not Chris Avellone.)
Update10MAY14:
1) They've now released the supposed final version of TSLRCM -
Version 1.8.3. (File package has been updated as of 07 May 2014.)
2) They also updated that
M4-78 EP add-on as well. (File package has been updated as of 07 May 2014.)
3) Severian Silk mentioned
elsewhere about compatibility issues with using the Jedi Temple mod and M4-78 EP. That's been fixed too with
this.
4) If you sincerely hate the Kaevee stuff and wanted the old conversation with Jorran (the shifty salvager trapped in room by laigreks) back in the game,
this unofficial tweak pack is right up your alley. There are other options in this too and you can change the things you want - it isn't a all-or-nothing type of mod.