But clearly something has happened, that they accept as evidence I plan to pay them. How difficult would it be for me to revert that action once I've left?

What has happened is that your transaction has propagated through the network, and some payment processor that they contract with has seen that transaction and confirmed that it has been sent out.

Since it hasn't been included in a block, it's not considered to have really happened yet. You could send out another transaction with a higher fee, sending the coins back to a wallet you own, and a miner could see that and decide to take the transaction with the higher fee over the lower one.

One thing that is preventing that from happening too much right now is that the default bitcoin software will refuse to relay transactions of coins that it has already seen, even if they have a higher fee, so it can sometimes be hard to do this in practice. But there's nothing that says this must be the case, in fact early versions did allow later transactions with higher fees to supersede older ones, and they are planning on re-introducing this feature as it will allow people to increase fees on stuck transactions if we start hitting the block size limit and transactions with lower fees start getting dropped by miners.

Or, even without this, you could have sent out two transactions simultaneously, one to the restaurant and one to yourself. If you happen to know which node it is that the restaurant is using (or at least figure out which nodes are topologically closer), you can send one transaction in that direction, and another to the rest of the network, and have a good chance of the one that sends coins back to yourself being the one that actually gets mined.