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Boundary layer fans are a thing, and they work. I had a closed back 12" GSO Dob, with just a cutout in the camp mat used to block off the back. The fan was then used to push air across the back of the mirror and exit up the sides. Just above the mirror surface I had a ring of rubber weather stripping attached to the side of the tube, the air would hit this weather stripping and be deflected over the surface of the mirror, ultimately making its way out the front of the tube.
I have seen other boundary layer fan setups where a small fan is suspended by wires in the blind spot below the secondary, it does not affect central obstruction and is invisible to the scope user. There are also solutions where small scroll type fans are used to blow across the surface, usually on custom made Dobs. I built a 10" Dob (Carbon Strut tube) where I used little pusher scroll fans from eBay to blow air over the mirror surface (they were attached to the carbon struts).
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