You can always check for overheating with your hand, the EVSE should only be moderately warm, from what I have read the charging cord and parts should all be able work in moderate rain. As for myself, I would never charge in the rain. I can say that because I have a garage. Anyway, welcome to Prius Chat!
I do like the CordPro solution for the extension cable. Now that rain season is approaching, I probably need to figure out something for the EVSE in the rain. The EVSE looks to be water-proof/water-resistent, but I would think it would help to extend its life if it was protected in a water-proof breathable bag.
