I just tried out something with my Unicron.
I took a few random large Headmaster heads and placed each of them on top of Unicron's body, in lieu of Unicron's own noggin. The heads of Titan class Scorponok, Titan class Grand Maximus (TR Fort Max mold), and Car Robots Brave Maximus (G1 Fort Max mold) all just sit very loosely on Uncron's body, as expected.
BUT, the Robots in Disguise 2001 Megatron Megabolt mold (the one based on Beast Machines Megatron's big floating head) actually sits rather snuggly on Unicron's body. I tried it both with and without the spider legs, and the legs actually help to secure it in place! The rear legs rotated upward line up rather well with Unicron's collar, the middle legs rotated to point backward wrap around the collar's front edges, and the front legs just rest on the top of Unicron's chest.
This means that one of (if not the) most accomplished Megatrons ever has a toy that can actually interact with the Unicron toy in a somewhat decent fit, allowing him to appear to possess Unicron's body, yeeeeesss.