Oh dear, so much wrong here
So many places where people have failed to think on this one:
1) The original failure. Why doesn't the mouse come with a charging dock in the first place? Every wireless mouse I've ever bought came with one. Never had a problem with running out of charge unless I've forgotten to put the mouse away for several days in a row (one of my wireless mice solved even this by having exchangable batteries despite having a charging dock).
2) A USB powered mouse charger? So the obvious assumption is the charger is powered off the computer? Except you want the charger powered when the computer is off and you're not using the mouse, not when the computer is on and the mouse isn't in the charger because you're using it. OK you can get a USB adaptor that plugs into the wall, but thats yet another thing to buy.
3) Why wireless? Whats wrong with a couple of metal contacts? Works fine on my wireless mice. A little more efficient that wireless charging.
4) If you are going for wireless power to the mouse then why use it to charge it? Why not power it directly? Put the charger in (or under) the mouse mat. At this point USB power starts to make sense too, since you need power when the computer is on (not when its off). Now take it a step further, if you've got a USB wire running to the mat to power the mouse then put the reciever in the mat too. Reciever is now much closer to the mouse (especially compared to a plug in the back of a tower under a desk), so less chance of signal problems as an added bonus.