I got lucky a second time, and found that Open Library has put up Galen's De Compositione Medicamentorum from an edition of 1530. I don't read Latin splendidly, but I can read it well enough to know I found the passage I was looking for:
Reperi solum ad compositionem idoneam eruginem et ceram: quibus acceptis, et ad ignem cera liquefacta cum oleo rhodino, ut liqui dum ceratum fieret, miscui cerati librae unciam unum eruginis, hoc est duodecima partem: statui enim decimam vel duodecimam temperare: suspicatus quidem majorem, ut acriorem cera futuram, minorem tanquam imbecilliorem.
Again, I've rather poor Latin, but it appears to be made just from beeswax, "erugino" (probably rust or verdigris) and rose oil (and for those who didn't get the memo before, that's vegetable oil infused with rose petals, NOT essential oil of rose) melted together -- no other emulsifiers, waters or preservatives, which makes it more of a pomatum. The addition of water and vinegar seems to be a Medieval innovation. We also lost the "erugino" which may be either a colorant or something believed by Galen to have another medicinal property (verdigris, for example, was once considered good for wounds.) This assumes I have a correct translation for erugino -- who knows, maybe it is a liquid?
This answers that question I'd long wondered -- namely, whether cold cream really was an invention of Galen's or if it was just attributed to him by later sources. It seems, from this, that the basis of the mixture is his, but it has come a long way since then with many recipes including changes and new ingredients like mineral oil, borax, rosewater, and more.
It may also be worth mentioning for these historical purposes that Rose Oil in ancient times could mean a more complex bouquet than simple roses. Pliny the Elder, writing about 75 years before Galen, described how Rose Oil of earlier times was "of the most simple nature, though more recently there have been added omphacium, rose blossoms, cinnabar, calamus, honey, sweet-rush, flour of salt or else alkanet, and wine." (Funny that rose blossom [flore rosae] would be a new addition, though perhaps he means the whole flower compared to just petals?) So it's not impossible that Galen's original rose cerate might have had vinegar or honey or some other ingredient via the rose oil he used.