T4 rear brake upgrade

I've not heard of a conversion, but your best bet would be to ask on the UK T4 forum - there will be someone there who will know if such a standard product exists.

I suspect that one of the things that will trip you up will be front/rear bias, because the brake apportioning valve (which I guess yours will have) will be wrong with discs on the rear, so it will not be simple.

As with lots of safety-critical modifications, the problem is likely to be as much about finding someone to sign off the resulting lash-up as safe (in legal liability terms) to use on the roads.

I doubt that the DVLA would be interested, but of course your insurance company will need to be informed, and may ask searching questions about the safety of the resulting system.