I've tapped and modified my valve cover to accept -10AN however, is it safe to rivet the oil baffles to secure them? Has anyone done this rather than threading?

 

If you have proper rivet then, Yes. If not it's best to use bolts and then safety-wired them.

 

I drilled the holes to 5/32 and have rivets at 5/32.

Is that what you mean by proper rivet?

 

Proper size (you already have) and proper material. I think you don't have a lot of options besides aluminum. Just keep in mind the magnesium alloy is pretty soft.
If I were to do it I would go with Stainless bolts and then safety-wire them.

 

i wanna see pics when you get done.
I have never seen it done...