The Wild Weasel Unit profile states that there are two hard points that can mount various modifications, but the only one mentioned is CP-03 Beam Gattling Cannon. So I was wondering, what are the other mods that can mount onto the WWU?

And even the CP-03 is mentioned with caution. The answer to this question is "That's for me to know and you to find out the hard way."
In other words, experiment. I'd advise small, low-powered weapons that can fit on a simple hardpoint, but something the size of the CP-03 would be pushing your luck something fierce. It's more'n half-likely you could have 'bad' results.