In skeletal muscle, which metal ions must be attached to ATP before myosin ATPase can function?

I really don't think any "metal ions" attach to ATP. The whole function of ATPase is to break down ATP alone. Anything added to ATP to facilitate its breakdown would surely be part of the catalytic reaction?

I don't know if this is helpful, but Calcium is heavily involved in skeletal muscle contractility. It binds to troponin C, which causes a configurational change in tropomyosin. This then exposes the myosin ATPase site I think, which can then catalyse ATP.

So in answer to your question (hopefully!), Calcium is the metal ion necessary for myosin ATPase functioning, but not by attaching to ATP. I don't think anything anywhere really attaches to ATP without it being part of the catalytic process of ATPase.