Valid constraints for non-binary references?

Hello, I'm having a little trouble figuring out how the constraints=~ options work/do not work in the context of, say, a Unif(0,1) reference ergm. Ideally I would like to constrain for the density of nonzero dyads or fix the degree of only a few high nonzero-degree nodes, but those sorts of options don't seem to work (or perhaps I'm just missing something obvious?).