Might want to get some temperature measurements before choosing to feed water and dirt into the back of an alternator. And since it has fans at both ends, the blast tube is supplying something less than half the cooling air anyway.
Then consider the source of the heat, because it's always more efficient to reduce the source.
First convection; we know typical air temperatures in the lower cowl ballpark at 120F to 180F. Not insignificant, but arguably no warmer than cars, where the alternator lives in the outflow from (typically) two stacked heat exchangers, one of which is operating at 215F+.
There isn't a lot of metal to metal contact in the alternator mounting, so heating by conduction isn't likely to be significant. That leaves radiant heating.
Look at the photos below. What red hot thing is 6" to 10" from the back of the alternator? Does your red hot thing have a truly effective heat shield?
It's one of the goofy details of Lycoming design. The put the alternator on the side near the #1 headpipe. They put the starter on the cool side, where the headpipe is quite a lot more rearward.
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