>If Sys(2007) returns a checksum value for a character expression, is there a way to go the other way and get a character value from a checksum value?

It's just a 16-bit CRC of the string, nothing more. Since it's 16-bit, there can be no more than 65536 different values of it, and the number of possible string values is pretty much unlimited (try just 256**16M for size), so there must be many many strings which have the same CRC value. Which one of them would you expect to be returned?