Pinebook Pro?

[GLSRacer]

How possible is it to throw in a really thin fan with a really basic heat spreader?

Not very. There is no need for a heat-spreader, since the bottom of the case makes contact the SOC and acts as a big heatsink/spreader already.

I've been torture testing the device and playing with thermals over the past few hours. While you can get the unit to spike 80*C under unrealistic load in under a minute, it doesn't down-clock massively (1.4-1.6ghz on big cores) when totally grilled.

I have played some games, watched 4K movies and done other demanding but real-life things and never hit the thermals. Playing Tekken 6 (PSP) was probably the most intense real-life benchmark, and that had the SOC hovering at 76*C, which is  just under the thermal threshold for maintaining full frequency.  

That said, I'll do a little bit of modding for fun in the coming days and see if there is something simple to be done to improve thermal performance further - not that I think its necessary.

Glad to hear that it stays fairly cool even under sustained load. I'd be interested in seeing if temps improve with a small uUSB cooling pad. I have a small dual fan pad that I use with my Y700 for gaming. I imagine that you might be able to drop temps by 5 to 10C with the fan cooling the bottom of the Pinebook Pro.