In most cases no, you couldn't use it.  I asked a buddy of mine about it, he makes some of the logo's NASA uses.  This is what he had to say...

"As long as he's not making money off it or implying NASA's endorsement, I think that's okay."

So yes, you could include it on a freebie prop.  But I'd recommend you put in the EULA / ReadMe for the freebie that it, and specifically the NASA logo is for non-commercial use only, any other use is a violation of NASA's copyright and trademark.  That means if someone wants to do an art render and share it here, that'd be fine.  If they want to use it to make a product endorsement, that would be a no-go.  If they wanted to make an image and then sell it, also a no-go.  If they took the NASA logo out, then its no longer an issue with NASA.

That's as close to the official word as I can get without running it up through channels at NASA.