I have 250 feet of 5/16" (8mm) HT (grade 43, or G4) chain spliced to 150 feet of 5/8" rope. Not sure how much more weight would be OK, but that amount of weight seems to be no problem. It does fill up the chain locker.
I think your 10mm chain weighs about 1.5x as much per foot. If you want a lot more length with not too much weight, I think 8mm G4 would be plenty strong enough. You would need to change the windlass gypsy wheel, though.
Another alternative might be to splice on some rope. That's what I did when I bought the NT, figuring 250 feet would not always be long enough for cruising BC and SE Alaska.
Dream Catcher (Nordic Tug 37-065) Poulsbo WA
Previously: Bounty 257, C-Dory 22 Cruiser
"Cruising in a Big Way"