I see what you are saying, this is kind of tricky!
I wouldn't delete the plug and go directly to the battery, that doesn't sound like a good idea at all!
If you want something dangled out the side, all the power to you. I would replace the USB port on it. You really don't need schematics for that. A lot of companies now a days treasure their schematics and will not release them.
When I first started soldering, I had a cheap 60V iron (I think it was 60V). I think it costed around $20, and I replaced numerous micro USBs with that. It really depends on how much room you have around the port. Some have capacitors and a bunch of other components near it, and you don't want to start heating up other components on accident or breaking anything. You should be fine with a cheap iron or whatever you already have. I have even used a wood burner because it had the curved tip before!