What is considered to be a safe, temperature tolerant finish to apply to a useable wooden mash paddle? The intention is to extend the life of the mash paddle while still actively using it, so the finish should be tolerant up to 170°F+ (to account for initial strike temps and the unexpected, accidental over-shot temperatures). It also shouldn't leach any flavors or contaminants into the mash while stirring at temperature.
The first thing that comes to mind would be to look for tabletop/countertop-finishes, since these have to be able to endure high temperatures of water spill. As far as contamination goes I do not know if they will release anything when exposed to the high temperatures.
One brand of high heat resistant counter top finish is Waterlox. There is no specification on what kinds of temperatures they can endure (but they do mention boiling water). The best would probably to get into contact with a local retailer to find out the limits.
EDIT: If you are looking to go for food grade safe I think you might have to ditch wood altogether though and look towards metal mash paddles.
I'd advocate for just using a harder wood (such as maple) without a finish. Some of the finishes out there may say "food safe", but the reality is that even at higher mash temperatures there is the risk of those chemicals leeching in over time. Depending upon your level of paranoia or concern about foreign chemicals and introducing them to your beer, I'd just suggest a mash paddle that's made with a harder wood and don't bother staining it. Clean well after every use and it'll serve you well.