Is it possible to make a checkbox for a pose bone or a constraint?

Yes, the simple way is to just use a boolean property, it should show up as a checkbox automatically.
Sadly you can’t use ID Props (the custom properties) to define a boolean, so if it isn’t a built-in boolean, you have to add it to the type, e.g.:

bpy.types.PoseBone.myprop = bpy.props.BoolProperty(default=True)

and then later expose that property in the UI.

This has the effect of adding myprop to every posebone, not just one – and can only be done via python.