There was a similar thread earlier in year. I can't recall the conclusion, as I didn't follow it beyond my post.
My recollection is that the E30 bumper lower half can't be bought with a removable panel, but you could as you suggested fit a colour coded GTI lower half with the removable element. Depends how precious you are about the E30 lower.
For what it's worth the GTI makes for a surprisingly good tow car. I regularly tow a 1200kg trailer, and the car is very stable. Surprisingly not too hideous on fuel either - 27 mpg last Friday on a 180 mile trip, with 1200kg trailer, and a car full of jerry cans full of fuel, tools, spare wheels and a passenger. Can't grumble really.
I've got a Westfalia removable, and it's seemingly very well made.
