Can a mud ring be used in reverse?
Here in Australia you'd probably use a surface mounting block over the top and then an IP rated outlet.

Mounting block is bigger than a round j-box. May need to seal between block and wall and then you're done.

Along these lines:



Source: http://updates.clipsal.com/assets/images/products/A00047211.PNG

Not sure whats available and standards in the US but if surface mounted over the top and larger, shouldn't be an issue but uglier than flushed.

May be able to drill/chase in a large wall box, however it's quite a bit of work.