I've added a number of map screens for each player (labelled Blue 1-5 and Red 1-5). I've also created a number of working pieces to use with these maps, as well as a set of pieces which can be made invisible to hide them from the other players (in theory). Unfortunately, the pieces are still visible to other players when they're supposedly invisible, so I'll need to try to fix that. Probably I'll need to define a separate set of pieces for each player which only that player can influence.
The next step is to work through the programming tutorial here:
http://www.vassalengine.org/wiki/Programming_Tutorial and seeing if I can apply any of this to automating some of the actions, such as revealing and hiding the pieces. I'm also going to redesign the layout and size of the different map boards and add some more pieces.