In the old days with cards you might have the option of “if” moves that would speed the process up.
One danger of “if” moves happened to a guy who used a Q-side fianchetto defense. His opponent went 1. e4 and the guy’s response was 1 … b6 2. if any Bb7. The opponent sent 2 Ba6 Bb7, 3. Bxb7. If my memory is correct, the guy resigned after going down a Rook on move four with no positional compensation.
Someone I knew a few decades ago told of a similar story: “Pleased to meet you via postal chess. In response to any legal moves I will play 1. N-KB3 2. P-KN3 3. B-N2, and 4. O-O.” The reply came back: “Please to meet you via postal chess. My first four legal moves are 1. P-K4, 2. P-K5, 3. PxN, and 4. PxB.” White was never heard from again. 
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I suppose one could send something like:
if 1. …e5 or …c4 2.Nf3
if 1. …d5 2. exd5
etc. through all twenty possibilities. 
For that matter, if you have a forced mate in 3, Brian, you could send conditional moves on all possible remaining moves? (Thus, if you’ve already delivered move 1, on receipt of the next move you can conditionally play out the mate?) Of course, we can’t review the position until it’s ovah. 
Yes but he might not realize it is mate in three.
On gameknot you can set up a conditional move queue. So you don’t have to watch the mate. He does.
To Xp; it is unlikely that the other player is seeing this exchange. And in any case BL could be wrong as well 
I feel your pain. I just had a time forfeit win when I was the exchange and a pawn (on chess.com). It does seem like bad sportsmanship to me. Just resign or play on
After a week, he played out the three moves to mate within one day.
If he had visited this forum, he would still be playing.