Chris, if the reason you want to avoid gelatine is because it's an animal
product, consider trying kosher gelatine. It's vegetable and might give
some of the effect you're after.
I do not, incidentally, recommend one or more coats of gelatine on top of
a print as Luis does. In my experience, at least, it degrades rather than
enhances the print by putting a haze over it. Of course it could vary
according to the paper and the process, and be better on prints that are
not gum.
But you didn't say what paper your dried dull salt prints are on... a
blottery, or overly absorbent, paper could be the problem. Consider trying
a harder and/or better sized paper.
Judy