Yes I have. All "headers first" does is save bandwidth on re-transmission. In order for other nodes to build upon your block in a trust-free manner, they still need the whole block. Edit: there is some provision for blindly trusting the header, then banning mis-behaving hosts later.
You are correct, it's not implemented and full blocks are required to trustlessly verify txes.
However, what I'm talking about is simply postponing the upload of the txes for the blocks you mine.
If you're a miner and can't find a way to connect the inputs of a new tx you're receiving, you just ask around for the merle branch you're interested in, or postpone inclusion of that particular transaction. And when the previous transaction comes around, you can include it.
Miners will have an incentive to push such a behavior, because it'll allow them to include more fee-paying transactions, reddit would also push it because it'd allow them to keep tipping each other bitcents onchain for a satoshi per transaction.