There's no description or outline lines on the TRF mapping (Ray are the TRF doing mapping on the green roads project for IOM?)
screen shot shows the green roads with an +++ Byway, I would imagine there would be further information displayed at the entrances of the green roads if there was a TRO closure on them
Stan Watt from rotor adventures in Dorset does a tour on the IOM in TT week, he'd have information on routes or a file
That map on the link is probably the best resource, you'll have to make your own gpx using that info unless someone else has already done so. That is an official map because they know they get a lot of trail riding visitors and actually provide something useful. Other local authorities should take note!
I do have the gpx file for the TT course if you want that.
There is no IOM TRF group, and it is not included on GRM (at the moment)
I've been a couple of times (for the TT so on a road bike) and was pleased to see finger posts with both a horse & rider and trail bike indicating we were welcome. The Isle of Mann is a very enlightened place.