Your case seem very rare. You want to have a 4-of-6 multi-sig, but if 6 out of 6 have signed the transaction, you want it to be shown. A quick solution that comes in my mind is to create another 6-of-6 wallet (with the same xpub keys) and move the money from the 4-of-6 address to a 6-of-6's address. If you did that, then in the blockchain it'd be clear that all of the co-signers of the 6-of-6 address agree upon that transaction.
I think OP's point is for redundancy, in case of certain disagreements so at least 4 of 6 of the parties can agree on it and the approval of the final 2 isn't needed.
Wallets should probably discard excess signatures for obvious reasons, because it is quite rare for people to have to prove that all of the parties are in control of their keys. Would it be possible for you to just create 2 transactions, with one of the parties signing both TXes and the other parties being unique.