idea yes cheating

character number: 2 bytes
replaces: 4 bytes


2 B64 chars: start pos (up to 4095)
6 B64 chars: 4 replacer bytese


3 or 2 B64 chars: start pos (up to BIG HUGE)
5 or 6 B64 chars: 4 replacer 7-bit ascii codes
or or 1 [ unused bit]
1 bit: splitup shit: 0 = 2 + 6, 1 = 3 + 5

chars used will be:
A-Za-z0-9_ and -

so a code might be like
A0-rH_5c