Borage and Comfrey are the first couple that come to mind for me. Careful with the comfrey, it can get outa control. The 'bocking 14' cultivar is sterile, but still spreads readily by roots cuttings, purposeful and otherwise.
Burdock will throw up some big leaves, and has an edible root (known as gobo root in Japanese cuisine) and stem. As a biennial, you can get of it, should you ever decide to, by cutting back before it goes to seed, and by not rototilling the substantial taproot - the pieces will regrow.
Castaway Compost - Yer Trash be Treasure! castawaycompost.com
Location: Victoria BC
posted 3 years ago
Also, I would vote borage over comfrey since it's an annual.
Fair enough, comfrey is definitely something of a once-only choice! Despite this, it's so useful that I wouldn't want to do without it; I consider the very... persistent... nature of it to be a feature not a bug.
Conveniently, though borage is an annual, it readily self-seeds, so you have it as long as you want it, at least around here.
But, now that I think about it, if the soil is great, then maybe squashes/pumpkins/melons? They may try to escape their bounds, sending vines around, but you just chop off the strays. And they'll definitely make lots of leaves. But the clincher is, if they set fruit, they'll make many POUNDS of compost. Much more weight, I expect, than anything that will be making only leaves.