Provided the closed graphics driver is probably not glib dependent, a completely OSS Fremantle is theoretically possible.

No idea about things like telephony framework, skype, etc.

Also while I tried to port a newer Clutter version to Fremantle some time ago, the glib version was one of the main blocking issues, preventing Clutter past 1.4 from running (together with some missing EGL header files). IIRC Hildon is currently using Clutter 0.8 so upgrading Clutter & making Hildon use the newer version might be also useful, while not easy.