In my own, haphazard way, I've been slogging away at the Daggerfall data files. After writing my own stuff for reading data as described on the file formats/hacking section of the wiki, I tried to fill in the gaps for the unknown quest opcodes (Hex/Dec):
05/05 - When Item dropped at Location: Set state
1B/27 - If state: Add Location to map at specific coordinates
23/35 - If state: Set multiple states (usually used to set Globals)
31/49 - If state: Cure PC of status
32/50 - If state: Show Video
3F/63 - If state: Add Mob as Escort
40/64 - If state: Remove Mob from Escort
47/71 - Give gold to NPC: True/False
48/72 - If state: Inflict Status on PC (disease?)
4D/77 - If PC Level >= X: Set state
4E/78 - When NPC talked to: Set state
4F/79 - If Faction Reputation >= X: Set state
51/81 - Unlock Location/NPC's Location (not sure of this one)
53/83 - If state: Teleport PC to location, specific coordinates
57/87 - If state: Place Mob at specific coordinates
If anyone's still interested in hacking this excellent but buggy old game, here you go.
Edit: Oh, I also wrote a program for decoding the 'message hashes'. Naturally this is sort of slow, but by passing the possibilities through a dictionary filter it does produce some quite workable results. I've done a bunch up to 7 letters and produced my own 'plainishtext' version of all the QRC/QBN files. If anybody is interested in having a look, let me know and I'll pop them on my webspace.