My attempts to post this reply in your save game editor thread have been met with mysterious timeouts, so I'm forced to resort to this...
What I wouldn't give to see the documentation for these files...
The file format documentation for ERF and GFF is on nwn.bioware.com in the For Developers section. The SAVEGAME.sav file is an ERF that seems to contain blueprints for party member selection (augmented by partytable.res, also a very simple GFF file), an inventory, and an ERF for each open module. The savenfo.res file in the savegame directory is a GFF which contains various information including the current module. The module.ifo resources contain the majority of the relevant module state.
The actual fields of many of these GFF formats are not documented, but their functions are easy to discover by applying a bit of common sense and experimentation.
So, the simplest way to get at the current party members: Read LASTMODULE out of savenfo.res, extract the named resource (ResType 0x0809, IIRC) from SAVEGAME.sav, and then modify the structures in the Mod_PlayerList list in module.ifo (it's pretty self explanatory - ClassList for classes, FeatList for feats, Appearance_Type for player model, etc). No searching required.
I'm not sure if this is old news to you or not, but I didn't see any mention of it.