Thanks to everyone for the responses! Interesting perspectives 
She gets overheated when I run her UNLESS the temps are cooler (under 50 or 60), so yes - I’m looking for something to put on her right after the run to facilitate cooling…
I usually put water on her, sometimes a cool, wet towel - and obviously encourage her to drink.
I run her behind my townhouse in a grassy area on a long line; just use the Chuck-it and have her chase and retrieve back and forth. If the weather is cool she recovers quickly; obviously I try to run her during the cooler times of the day! She’s white, but that doesn’t seem to make a difference (I have a black horse, and in her case her coat color obviously does NOT help in the summer sun!) My last Whippet was also mostly white and had NO issues with the heat; he was “Mr. Cool” in general 
I could conceivably put a wading pool in the small TH backyard (we can’t leave stuff like that outside, alas), and put some water in it for “post run”, but I thought a wetted cooling coat might enable me to help her cool down - maybe while walking her in the shade after a run.
Alas, our outdoor spigot does NOT work :(, so I would have to drag the pool over to the spigot (next to the backdoor) and fill it that way. This is probably why I haven’t considered doing this in the past! I usually bring her inside and put a cool wet towel on her, but immersion would obviously be better.