My guess is that the problem is reliability. NATS need a system that will always produce valid data. If NotamPlot doesn't display it correctly then they can shrug their shoulders - if they don't do it then they may be liable for the consequences. I suspect this is the main reason..?
If you looked at the Farnborough Airshow TRA NOTAM last year on NotamPlot, then it produced a small circle centred on Farnborough - however, the TRA was much bigger (and it wasn't circular!). To do it correctly you need some sort of self-defining format that can encode every possible TRA (XML would do nicely...
) - it's not impossible by any means, but it's more work than just publishing/plotting the Q-line.