Turns out there were others. A little searching before I posted turned up one that had been at SMU and another at Emory. The MetaFilter discussion turned up mentions of such monuments at Hobart College, Tufts, and Middlebury College. It seems very likely that there was/is also one at Babson College, which was founded by The Foundation's own Roger Babson.
So far, I've seen two different texts for the monuments:
"...to remind students of the blessings forthcoming when science determines what gravity is, how it works, and how it may be controlled."And
"...to remind students of the blessings forthcoming when a semi-insulator is discovered in order to harness gravity as a free power and reduce airplane accidents."I think this may be the start of an on-line collection.