Dude I totally understand where your coming from. we have owned many horse boxes and also built a few for our selves as well and understand the point you are making. But to be honest this problem if we look at the big picture is quite a small one. I was reading in the horse and hound about police camping out and waiting for horse boxes leaving county shows during the summer with a view to clamp down on the amount of over weight horse boxes on the road. To be honest the statisics have shown that with 7.5ton lorrys (which are the most popular with horse owners) only 1 in 20 where under there 7.5ton limit with two horses on board. The real issue is that when a 7.5ton lorry is converted into a horsebox it first of all needs to be of full width (7 footish) and of a decent length. After you have made a living area with all the stuff that goes into it ie, cooker,fridge, double bearth sleeping area etc then have a toilet, shower , storage areas for your tack , a decent amount of water , all the metal work for the partitans to seperate the horses and of course the ramp which has to be of a decent construction to allow the horses to run up and down it the weight quickly adds up.
So before you even put a horse in your lorry your in most cases very close to the weight limit.
This I feel is a more pressing issue than not having a tilting cab, as with your example the lorry wasnt able to be fixed by the side of the road. In most cases if a lorry breaks down on a moterway it will have to be towed to a workable area anyway to get fixed as i doubt you have all parts to fix all problems for all lorrys in the van with you lol.
I understand your concern about not having correct access but i feel we need to look at changing our attitude to horsebox construction in gereral. Having a lorry that will be over weight if you put a horse in it makes it a motor home unless you wont to potentially lose your liesence lol.
But please understand horse owners who have a requirement for a lorry will only buy whats on the market ! We dont all have the ability - space - knowledge to build our own.