It is just a stupid question. Imagine if I start mining over an old block which already has many subsequent blocks, find a new block and submit to the network. Will they accept it and create a nonsense fork?
I know it is the longest chain that really matters, but would the network accept my block, creating the new fork or simply reject it?
Only work for the current block is sent out. If you submit a hash for a past block it will be rejected, probably as stale or not meeting current difficulty.