Yeah, a threadgroup which contained individual threads for PCs would be ideal. With my group that recently finished, I gave them a scanned copy of the chronicle sheet, complete with my signature, but they were on their honor to fill it out correctly. With something like this, the chronicle sheets would have to be scanned to the players upon completion of the scenario, then filled out by hand by the player, then rescanned back to the PC thread. GMs and the Venture-Captain could then verify that everything is above board every time the PC starts a new scenario.
I noted this in my previous game, and I'll say it again here, because I don't want anyone to be confused: once you have a PC join a scenario, that PC cannot be played in any other scenario (including at home with friends or at conventions) until the scenario they are in is complete. That's very important, because your PC will be "in the field" for a while, considering the pace of PbP. My group here on the Weave finished a scenario in a month, but that translates to only one-third of the way to leveling. You level every 3 scenarios.