Right now we have ToToken and ToString. Neither of these properly preserve the prefix property of cell IDs: the only format that does is binary. It's possible to encode this, either as a binary string, or as some encoding that encodes each pair of bits in the string.
Eh, @DennisOSRM is right, this isn't worth it. Range queries are doable with rangeMin and rangeMax.
The route would be using the lsb though: https://github.com/mapbox/node-s2/blob/master/geometry/s2cellid.h#L279-285