I think a lot of it might depend on how tight the engine is in the first place...and the quality of materials used in it's manufacture.

If you have an engine that is a typical "sport" type engine that's designed for bashing it's probably got less of a pinch than a higher quality race engine, and will withstand some "abuse". (they are designed with the knowlege that there will be some people with little or no previous experience operating them )

I wouldn't want to run a new and squeaky tight (read: Novarossi, or RB, or engines of that sort )engine excessively rich for very long...just about as long as it took me to tweak the needles and lean it some...

I've run ringed airplane engines so rich that they needed the glow plug battery connected to stay running (even @ WOT ) but that's another story, aye!?

To try and answer the O.P.'s question though...it can prematurely wear the piston, but at the temps you are talking about I don't think you need to worry much.

Just one example...don't know how relavent it is here but...
I had a Thunder Tiger ,46 Pro (airplane engine ) that I flew year round, and that includes 15 degree winter days, snow storms, rain, etc.
It didn't like to be throttled back for long when it was really cold out, because even w/ a hot plug and more nitro, it would quit if given throttle on a "go around", until I wrapped alum. tape around the cyl.
I measured cyl. head temps as low as 110-115 degrees...and that was @ WOT leaned out.
I flew the snot out of that engine...put about 10 gallons of fuel through it before it started loosing power. (that was because it sucked in some dirt and the Nickle started peeling, but it still had a little bit of a pinch left at TDC believe it on not![8D])