A common way of committing some data to the chain is through an OP_RETURN with some hash. Doing this marks an output unspendable, but the output still requires some footprint (and cost) on the chain as it contributes to the TX hash.
So I'm wondering if it is possible to embed some data in an OP_RETURN in a script of a TapLeaf which is never intended to be part of a spend. If we assume that the Taproot output is spent using the key path, and it has already confirmed, it should be possible to later reveal the paths and scripts containing the OP_RETURNs, and it could be proven that the TapTweak hash did indeed commit to these hashes via the TapBranch hashes.
A single transaction could commit to a large number of OP_RETURNs which have no on-chain outputs, but whose hashes are indirectly committed to the chain. These could be revealed individually to whatever parties they concern. The paths and scripts would have no presence on the blockchain.
Maybe this doesn't even require the OP_RETURN as you could just use the TapBranch/TapLeaf hash itself as the data you wish to commit.
Would this not significantly reduce the cost of services like colored coins, timestamping services, etc, which commit their data directly onto the chain?