My .30-06 featherweight and .243 Win. compact Winchester M70's have black plugs where crossbolts may normally be found. Do these in fact cover crossbolts or are they purely cosmetic/

I have a Win Lightweight Carbine and a Win Lightweight and both have the epoxy "cross bolt". There is not bolt in there just an epoxy stiffener for a crossbolt.
In my Lightweight 30/06 I've shot near max 180 grain loads in it for the guns entire life (since 1986) and no broken stock yet so it might be working, this little gun kicks "hard" so I think it was an effective economical solution to stiffen the recoil lug area.

there is a wood screw under the black plug

I am sure there were production changes made that make you both right.

If there is a screw under the lug, is it epoxied in or just tightened and covered with the plug?

I have a stock that I would feel a lot better about if it had a real crossbolt rather than the plastic plug. Or a wood screw even.