Zapping a wand of polymorph at Master skill gives you control over the form the target monsters polymorph into, whether that is yourself or others. Other monsters still get their chance to resist the polymorph entirely.

It would not give you control over what an object would polymorph into, for obvious reasons.

However, if there are monsters that can use the wand of polymorph at master skill, this has the potential to become very annoying very fast, since they would be able to turn the player into a sessile monster, make them break their armor, or turn themselves into something very threatening and out of depth.

When you attempt to polymorph an item that has polymorph magic (wand/spell/potion of polymorph, tin of meat that may cause a polymorph when eaten, etc) it explodes and creates one or more monsters affiliated with polymorph magic, such as polymorph vortices (see YANI #3669), genetic engineers, or similar.