MongoDB Zone Sharding without min & max key
Your collections need "key" (like 'zone') what is set when documents need to be "zoned". There will be only so many different key values than you have shards. When user wants to be zoned, you update all users documents (at all sharded collections) with that "key". Those collections are sharded that zone key. You choose one of those shards to be "default" shard so, all those documents what don't have zone key set, goes there. This default shard have minkey and you choose some other shard have that maxkey.

Not the best solution, but...