IMHO, if the fixture is inverted, exposed, and outside...it will require a lens. If it's a damp location, then it must be listed as such. The thermal should not be an issue, as heat rises, and if the fixture is inverted, it should not be affected.
If it's inside, a thought should be given to the accumulation of dust, as it can ignite from heat generated by bulbs, if not cleaned on a regular basis.
(Had a fire in a retail store; track heads (LV) shining "up" behind a wall rack display; 2 months of dust bunnies, and up it went)
Were the fixtures on the plans; and if so is there a spec??
Sounds like the "cheap way out".....