Ok so this question was just posted to the LDrider list and here was a long time C3/SRC owners response straight from the horses mouth:
"A bit of bad news on the C3 bluetooth configuration. *It is impossible to
charge the radio collar in the helmet while it is in use. *Plugging into the
USB port suspends its operation. *I researched this at great length, and
spoke with engineers at Schuberth and Scala (which makes the Schuberth
built-in radio unit), and the guys at Bob's BMW also made a bunch of calls
to confirm my findings - there is no way to charge and ride simultaneously.
There is also no way to exchange the collar battery in rally-mode (requires
a bit of helmet surgery)."
This is a disappointment to me. :-(