I have towed Celtic Longboats before down at our local yacht club (google search comes up with this http://www.madocyachtclub.co.uk/page9.html
for info) and they do not like planing. What we have found is that they're very stable upto a point, then they try to plane, and consequently try and go into a capsize. To stop it weaving all over the place, have someone sat in the back on the rudder and it's fine upto around 8knots.
Personally, I'd never try and tow anything like that at 20knots, since any minor adjustment could result in a capsize, and you wouldn't have time to slow down and recover it before it goes over.
I have seen Pembs Watersports towing a line of around 6 or 7 toppers on the plane before now and it does look very good without anyone in the boats, although they do not look at all stable.
As annoying and frustrating as it may be for a rib, you may be stuck with a sedate 6 or 7knots for the journey I'm afraid