Hi Chris. I agree. I was going to say it has been done....but, on reflection, not quite in the same way as your mock up.
Kunstencentrum Belgie sold a 2006 T-shirt for the 2006 Hasselt event which reproduces the "Marty Tree" motif at a smaller scale than your vision on a chammy/cappuccino-coloured T-shirt though. They seem to still be available in certain sizes here:
http://www.kunstencentrumbelgie.com/programma/varia/triffidsthirts.htmlIncidentally, if you check out the "Te Koop" shopping section of that website there are still a range of limited edition photographs produced for the 2006 exhibition on sale.
Then there has been the "Never Trust A Triffid" Domino-produced Dave design facsimile. That used the tree motif but only in miniature on the sleeve:
http://www.dominorecordco.com/uk/releases/29-01-07/never-trust-a-triffid-tshirt/I'm not sure whether they're still available or not.
Others have been sold at the ASITSOAS shows, but I'm not sure what was available exactly.