Jesse,
I am assuming your engine is already broken in? This removes that from the equation. Second if run at a normal 70% - 80% load often, and idling in-between to charge batteries, I've simply not seen bore glazing be a real or quantifiable issue, at least in my customer base.
Can it technically happen? Absolutely. I think we tend to miss that when charging, the engines are loaded pretty well, at that specific HP, so we are not just "idling with no load". While the load may be small and the engine may not warm up to operating temp in cooler waters if you then go our and run her hard after this you should be fine four thousands of hours. We are already at "thousands of hours" so know this to be true for our engine based on our real world results. I have zero issue running our engine at idle when needed and when it does not disturb or impact others.
Do I believe bore glazing can happen? Sure, but I see engines die far more often from lack of use or other maintenance issues rather than the cruiser who idles to charge and runs her hard when not charging..
It is also advised by almost all diesel generator manufacturer that they be "loaded" to approx 70-80% when run, and in the real world, that also rarely happens.
I have one customer with 6k hours on an old 8kW diesel Westerbeke generator. It has an old 50A Heart Interface battery charger that is the primary use for the gen set. They occasionally load it for hot water or the microwave but usually the 8kW generator runs powering a small 50A battery charger. Even with every load on their boat drawing constantly it can't even come close to the 8kW or even 6.4kW which would be the 80% recommended by the manufacturer. They have been lucky to load this gen at 30% and yet it has 6k hours on it and still runs just fine.
There are also the thousands of owners out there who run Sea Frost engine driven refrigeration, as our boat has, who do not have issues with bore glazing.
About three years ago myself and a Beta dealer here in Maine had a long discussion with Stanley of Beta Marine, at the Maine boat builders show, specifically about bore glazing.
Stanley's take was this is a "non-issue" for Beta engines. He insisted they have not seen a single case of bore glazing in a Beta / Kubota block. Not a single one (as of three years ago). This is the US distributor of Beta engines and a guy who has been working with diesel engines his entire life. He is one of the more honest guys in the business and if we had to replace our Westerbeke tomorrow the guy who would get the engine sale....
He also admitted they have lots of customers who idle to charge. I suspect a lot of this is a CYA from the manufacturers point of view and with some engines there is more truth than on others..
The Universal M-25 is the same block Beta uses...... He did say that on some older Volvo's, some Yanmar's and some Vetus/Westerbeke/Mitsubishi blocks that this can be worse. That said we have a Westerbeke/Mitsubishi that has zero bore glazing and more than half its life spent idling. You simply don't get to 3600 hours on a 2002 engine, even when cruising full time as the PO's did, without idling the engine. They had no gen set and only had an 80W panel and a 50A stock alternator. The trick is after idling the PO's, and us always, run it hard.
I think the worry over this is a tad overblown if you otherwise care for and use the engine properly...
So can it happen? Yes, in certain cases I think it can. Does it happen regularly? Not that I have seen or heard of and almost all my customers, being on moorings, idle to charge....
