Substitutes for egg for a vegan
you don't need to use breadcrumbs on the polenta at all in order to fry it successfully...in fact i think it would be better without them and you will get a nice, nice browning from the frying...will you be putting something in the polenta? i put in fresh rosemary and sun dried tomatoes quite a lot since i like that combo..also asparagus and parmesan, but since your friend is vegan, that won't work for you...maybe a vegan cheese substitute, but some of that stuff is horrid, so be careful...
food is like love...it should be entered into with abandon or not at all Harriet Van Horne
Addressing your question squarely and from a technical standpoint -- dip the cooked and cooled polenta bars (or whatever shape you're cutting) in seasoned soy milk -- or better still, seasoned soy milk mixed 50/50 with a vegan, sour-cream substitute -- allow any excess moisture to drip off, then roll in crumbs.
Is there such a thing as vegan panko? If so, that's probably the "'A' Number 1" crumb choice.
If you want a tighter adhering crust, you have to control the moisture from the polenta by lightly dusting it with corn starch before dipping into the soy milk. You want a very thin coat with no excess.
If you want to try something more interesting than bread crumbs but not as plain as simply pan frying or grilling, how about tempura? Again, start with a light dusting of a fine starch or flour; make a thin batter from a mix of rice flour or corn starch, regular flour and club soda; and use the batter slightly rested and ice cold.