The source for the revBrowser external is here:
https://github.com/runrev/livecode/tree ... revbrowser
I suspect it'll require an architectural change deeper in the engine, however, to do what you want. Right now the external merely designates a rectangle within the window, control over which is passed on to the host OS's browser engine. Since the drawing is handled externally from LC, the LC engine has no way to buffer its contents in a way that would allow layering with native LC controls.
That said, a new browser control is on the roadmap for down the road after some higher priorities are completed:
http://livecode.com/community/roadmap/