If anyone is interested I "think" I've got a jig for coasters that works quite well and, so far, has resolved the problems listed on this thread. I've only had the opportunity to print a few dozen coasters today but the early results are very promising. I have explained below what I have done with this, if anyone else is interested in my approach. I went down this route:

1.) I had a jig machined from 10mm aluminium sheet. Metal seems to be the only real choice as MDF will just warp over time. However I didn't want the flimsy UniSub metal jig, as I want the jig to hold the coasters so they can be pre-loaded whilst another set is printing in a press.
2.) I had four "locating holes" drilled at each of the four corners, these allow a small "pin" to be inserted.
3.) I print my coasters at 12UP to an A3 sheet but can also be printed on a wide format machine easily. I also print four dots, one in each corner, which correspond exactly to the locating holes in the jig.
4.) To ensure registration I just need to poke a pin through two of my printed holes, remove the pin, turn the sheet over and then poke the pin through the holes in the paper and into the "locating holes" in the jig. The coasters are now registered to the image (even if the sublimation printer does not print the image straight on the sheet). A piece of tape is then stuck to each corner.
5.) The holes for the coasters themselves are machined to 3.5mm deep (my coasters are 3mm deep) and then into each one I have a bit of high temperature foam rubber (about 1.5mm deep), which allows me to get even pressure over each coaster and allow them to sit proud of the top of the jig itself.

So far, so good. Top quality print all over and very good registration, really quick to setup a set of 12 coasters to print. Once I'm certain this setup works well I will be arranging for some more jigs so they can be feed continuously.