Most epoxy putties can be thinned to a slurry with alcohol (dunno about Milliput but GS, Aves, Magic Sculpt, and ProCreate can), so I'd probably try that first. This being an emulsion (thickened paint, basically) rather than a resin, shrinkage and durability might become issues in such an application. Depends on how thick of a layer you're talking about, I'd guess.
It actually kinda bugs me that they're calling this "Green Stuff" when it's not an epoxy and isn't even made by the company that makes Green Stuff proper. From a marketing perspective It's understandable (people use GS to fill gaps, and this fills gaps too!), but at the risk of being impolitic, the gaming community is kind of an information ghetto when it comes to modeling. Way too many (even longtime) gamers still don't know a hawk from a handsaw when it comes to even basic materials outside of paints, so calling something that's not an epoxy "Green Stuff" (it actually sounds a lot like a gap-filling acrylic primer) seems like an invitation to confusion.