Ok, I'll add a data-point to my own question (I really only posted because it was late at night and I figured someone may answer before I had the time to dig into my myself).
At any rate, it took a good 30 minutes on the phone and asking to be transferred several times - eventually to a supervisor - but the supervisor was able to confirm that Wyndham Reward points CAN be used for Tryp properties that are run by (owned? associated with?) Melia Hotels. But it requires you call in and speak to someone who knows it's possible, likely meaning getting to a supervisor in most cases, and they have to put in a special request and someone from either Melia hotels or the property itself will contact you to finish the booking.
A little disappointing as it's more time/effort and likely to take long enough that you shouldn't plan on booking a stay for the coming week or two, but I am at least glad the option is there.