I suspect the easiest way to attach something would be to tap those two
holes and bolt on a piece aluminum angle, to which my plywood table would
get attached. How difficult will it be to tap these holes? I've tapped
holes before, but never in cast iron. Does it go easy? Is there any risk
that I'll damage the top? What if I end up having to drill some additional
holes in the flange?


Cast iron is one of the easier metals to both drill and tap.