Come now, your criteria were:

"I need to very quickly compress these strings into the smallest space possible and also be able to expand them back again later as quickly as possible."

and

"My environment ... no compiler"

No mention of change control or process there - you wanted raw speed and little space used and you couldn't compile on the production machine.

But I can imagine the approach I was thinking of - spawning an external process - (and the communication with that process) would negate any speed advantages and I'm too lazy to prove it one way or another so I'll shut up and go away!