OK, a few minutes of research answers my own question ... A cubic foot of gold weighs about 1200 pounds. (!!!) Now, a cubic foot of gold coins won't be that heavy, because of the spaces between the coins. An ideal packing strategy will let you squeeze about 1000 pounds of gold coins into that cubic foot; I'd estimate that just tossing them in will limit you to about 800 pounds.
So, a backpack stuffed full of gold coins will contain 40,000 g.p. and weigh 802 pounds. And require a character with a 26 Strength to lift. (If the backpack were weightless, a 25 Strength character could lift it, but that last two pounds is evidently the straw that broke the barbarian's back.
) It would probably split open at the seams & fall of the straps immediately, I imagine. We probably should have a weight limit on these things as well as a volume limit ... I wouldn't expect a backpack to be designed to hold more than about 100 pounds-who's going to be marching with more than that? That'll store 5,000 gold pieces;if you need to carry more than that, use the first 5,000 to buy some bags of holding/HHH's, then go back for the rest.
A stuffed belt pouch contains 8000 gold pieces and weighs 160 pounds. (Yeah, I know, 160 1/2 pounds. Whatever.) But you should probably split your gold evenly between two pouches to prevent permanent curvature of the spine. In the belt pouch's case, actually, we have a 'full weight' listed on the table of two pounds. So a belt pouch containing 75 gold pieces (a pound and a half) would be 'full', by weight. Probably a pouch with more gold in it wouldn't hang on your belt any longer ... Anyway, it would be pretty inconvenient. "Hold on-I'll give you exact change. I'm sure I have a copper piece in here somewhere."