OK, after a bit of mucking around I'm giving up on Calibre Sync in favour of Calibre Library, as it a) connects via IP, not zeroconf/bonjour/avahi and as such will be able to be seen from the Internet once I configure my firewall, and b) it works with the calibre-server component (ie: as it doesn't rely on the wireless device connection component I don't have to do anything funky like using xvfb).
As an aside - I couldn't quickly figure out how to fire up Calibre via xvfb and also start the connection server. As such, that is possibly a dead-end anyway.
I haven't tried Calibre Companion, but from the description on the Play Store it appears to also rely on bonjour and as such doesn't appear a good option for headless, nor for general purpose access via the Internet.
Finishing off, if anyone is reading this without having seen the various pages out there for headless Calibre implementations, check out something like
http://alt236.blogspot.com.au/2010/0...t-boot-in.html
...and note that as chaley mentions, you can't daemonize the wireless device connection protocol, so don't attempt to use client apps that don't support hard-coding in the configuration of the Calibre server (IP : port and optionally user/pass)