Hi,
that was me ;). I also talked to Bastian Bloessl and we agreed that I will create a pull request to gr-ieee802-15-4 once I finish my master's thesis and have it all cleaned up. In the meantime, you can find my fork of Bastian's repo here:
https://github.com/fewu/gr-ieee802-15-4. The branch with my work is called css_phy. Currently I'm working on the synchronization for the CSS PHY. My previous work focused on evaluating PHY and MAC performance with the assumption of perfect sync. It's also not really complete yet (e.g. preamble and PHR are dropped in the receiver because their content is assumed to be known) and endianness might be wrong at some points. But I will try to fix that in the next weeks.