Thinking more about this--and I believe I may have a solution...

I could put "threads" on the end of the case where the USB port is, and then have a screw on cover for that area (that you could seal with an o-ring).

That'd maintain access to the charging port, while offering a weatherproof design for when the device is in use...

Will take some re-engineering of my current case design, but betting I could make that work...hmmmm...