This is likely a Rube Goldberg winner.
The only case where this would make sense is if you have all of the valves closed, except for one unit. And, that unit's water path is so narrow at some point along the way as compared to the pump size that the pump can't move enough water to cool itself. In this case, one would need to have a path to relieve that situation, allowing the pump to move enough water to cool itself.
If you've got an extra above water thru-hull, I guess it can't hurt to plumb it that way. But, if not, I'd want to see the math that justifies putting another hole in the hull before I'd do that. Or, I'd set it up with that valve capped off, close all of the valves except the smallest unit, run it full on, and measure the temperature at the pump until it leveled off. If it wasn't getting to hot -- I wouldn't add the extra flow just to cool it.
With the valves open, unless he is adding electronic valves, even if the units aren't running, the water will be circulating, so this could only be a problem with valves closed.
If it is a problem without valves closed -- get a smaller pump. But, I can't imagine that.
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