Hi,
One way to achieve this is to create a server-side RPC function which takes the long match_id as an input and returns some abbreviated match code using a storage collection with 2 columns: match_code, match_id. Client would call this function when match-making: this RPC would generate a new match_code and insert it in the collection associated with the internal match_id, then return the match_code to the client that started the match.
On the match join, a client would call another RPC with reverse logic: take match_code as input and return the long match_id to the client to pass to MatchJoin.
You may be able to do it all in one RPC and some details may differ depending on whether you are planning to have authoritative matches or not.
Hope this works for you,
Michal