Would pomegranate molasses (which I have on hand) make a reasonable substitute for tamarind (which I was unable to purchase)?

i agree w/ bigpan. my only focus was the other ingredients and especially the red wine and madeora. i think the molasses should do fine w/ those. Really, both the tamarind paste and the pomegr. molasses are very thick and very pucker-inducing citrus punch- packing so i think it's actually a good sub. As bigpan says, just go easy and add more if needed as you taste it cooking.....
BTW if and when you do seek out tamarind paste, the best product, imo, is a very thick (tar-like) paste usually found in a red and yellow printed plastic jar , in Indian stores. If you can only find the block or the jam-consistency tamarind, don't bother getting those. Indians tend to use it as a citrus sub that doesn't have to be refrigerated and which adds a flavor kick to food. If you have been served a sweet and sour brown syrupy mixture w/ pakoras, that is a tamarind (and date) mixture.

The pom. molasses will give an entirely different flavor - but one that will surprise you. Start with a little bit, then add to the taste you want. Then, instead of calling them "Dan Barber's Ribs", call them "Randi's Ribs". !