Pass thru should work fine, whatever your DVD drive supports, more or less goes out the spdif that same as it does for analog outputs. What rarely happens is the motherboard maker paying the license fees for encoders to the various formats. So your 7 channel receiver, and 7 channel (DTS or whatever) onboard sound chip, only works in stereo. Same is true for most non crazy expensive sound cards, chip supports all sorts of modes and formats, driver does not with no possible upgrade.
Dolby Pro Logic might work, as it is a pure analog signal, ie tricks done to and decoded from a stereo analog signal passed between devices, vs digital formats.
No harm in trying it, maybe works fine for your needs, or even if it doesn't actually work, it still may be ok.