Should I be able to mount and play .iso images from the MVP? They appear in my video library (and can be played) on the PC but not on the MVP?
Not at the minute. This is due to the MVP only being able to play Mpeg1/2 files. Now granted VOBS are are Mpeg2 files but if they have audio other then mp2 then the MVP cannot play it.
Sub filters the directories to what can be played/viewed,
That seems fair enough.
I have also noticed it won't let me play divx/mp3 avi's on the mvp (or it would if my pc wasn't so slow) but the Hauppage software will why is that?
Hauppage's software will play the video it in its native format on your PC and do a real time transcode to Mpeg2 for the MVP. This transcode process in thier software is very system intensive on your PC's CPU just to get other file types on the MVP. The MVP can only play Mpeg1/Mpeg2 natively. This is why GBPVR does not do this, it is a separate process of the software.