don't think LR do an inertia lap belt for teh P38 however Exmoor trim do replacement seats for defenders and they advertise a floor mounted inertia lap belt.
No idea if these would fit (can't see why not, perhaps only issue would be where to mount the inertia reel assembly).
maybe worth giving them a ring
What are the ones like on the L322? if they are inertia, perhaps one could be retro-fitted to the P38
And I suspect you're not alone. Unfortunately Mr. jameslaneend appears to be one of those people who'll join a forum to ask a question, get a few reasonable suggestions and then bugger off, never having contributed and never to be heard from again.
You could try sending him a PM to ask if he did the conversion in the end. He'll get an email alert which might bring him back and then he''ll read this!