I have had the same problem in the past. The design we had was the home drive,apps drive and 4 data drives were mapped by default, then application specific mappings would each have a letter associated with them. Eventually though, we had more applications than drive letters.
The way we got round it was to duplicate letters for applications that were not used together - e.g. Finance people were unlikely to use a high resolution mapping tool that we use.
I know it is not a solution but I'm afraid it is all we could come up with