First time I've heard of this problem happening, but it's exactly the sort of thing that concerns me when suggestions of portion working are raised for things like Voyagers, IEP▸ and class 175s, none of which have corridor connections on the ends.
Portion working is such a useful means of operation that I believe all trains that can should be built with corridor connections, and those that can't (I assume this includes 125mph stock) should be built in long enough formations that multiple-working is not necessary in service.
You have portion working on the non gangwayed 140mph class 395
HS1▸ services though, which split at Ashford!
It shouldn't be a problem though with suitable information screens. The set-up used by the likes of Southern and Southeastern should be adopted by
SWT▸ , along with appropriate guidance before the passenger boards the train in the first place.
By the way, portion working does take place on
FGW▸ , it just isn't advertised in the timetables. I know of a morning service from Paddington to Banbury that actually splits at Oxford and the rear forms a service to Morton-in-the-Marsh and there's an evening service from Paddington to Oxford that splits at Reading with the rear going to Bedwyn.