Probably, I am simpy too much obsessed with both JA and DLS and it is all a pure coincidence but I couldn't help noticing that the name "Pomfret" also occurs in Gaudy Night.

If JA invented the name "Pomfret" - and I suppose this is the case since the castle in question is actually named Pontefract Castle as Carolyn has kindly explained to us - DLS might have intended an allusion to The History of England.
I guess, we'll never know and it is likely to be a coincidence, anyway, but I like the idea that DLS read and enjoyed this piece of JA's juvenilia and happened to remember the name when she wrote Gaudy Night. That would be so cute! ;-)