Well, they're several nav chart mods in the mods forum, why not look at those. I like the one that shows airbases as they change from US to JP, vice versa using flags. They're some old mods that put different charts on the table, but they really have no function. I assume you're aware of the better convoy shipping map mod for TMO/RSRD used with the radio.
Not sure how you would add more charts, obvious have to rob a function.
You may be able to use the F1 help screen and change the chapters to different charts. Someone just reloaded an old mod called Combat Information Center, that doe's just that, the chapter tabs are used for many different charts, moon phases, ect..It's a little old and needs some updating and can't be used with SCAF, but last I looked it was at the top of the list for gameplay mods.
You could place charts in your sub instead of the posters and such, but again no real function, unless you find something. Takes some converting, but I changed some of my posters to pictures of my wife, so you won't be seeing SS's at certain angles in my sub, really you can post them anywhere you have wall space...
You probably have found the mods so not trying to insult ya, but here ya go just in case, most are under gameplay, a few under graphics.
http://www.subsim.com/radioroom/down...hp?do=cat&id=2