OK, after searching through the petroglyph forums I found a partial answer to my first question: http://www.petroglyphgames.com/forum...showtopic=1735
Open up Campaigns_Underworld_GC.xml and search for "Heart of the Maw," which is the smallest one to start with. Copy all three campaigns (one for each playable faction) to another XML file and add its name into CampaignFiles.xml. Then just add and remove things as you see fit until you're satisfied with it. You'll probably want to copy and rename the files referenced by <Rebel/Empire/Underworld_Story_Name>, as they (well, their <Active_Plot> files) control the campaign intro text and hero spawns.
However I sitll don't know how to add a custom description/name - and how to open the mastertext.dat file.
PS and PPS also remain...