What representation are you planning? My thoughts about it tend towards DER-inspired formats, something like this:
for values small enough to keep the exact value in the index: <0x80 + sign*(#bytes needed)> <those bytes of 2's complement value> for larger absoulte values: same, but for the sake of speed read the value as hex, so we only need read the first digits (skipping initial zeros). for huge values, something like: (value < 0 ? 0 : 255), <0x8000... + sign*#length octets> <sign*length...> <initial part of two's complement value-as-hex>
(And if there is going to be a slapd.conf-option, that could just as well be an option with the max index-key size.)
Too complicated?