Maybe some kind of NFC device that you could wear so that the computer would lock/unlock when you were near it ?
The interface could just be a python script reading for serial over USB. I built some Raspberry Pi based RFID terminals that ran a python script in a loop, no problems running them for days on end.
That's the extent to which I know, someone else would have to chime in on exactly how to interface the script with access control.
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---going further---
Then just encrypt the home folder's contents on a file-basis & sync it with a privately owned server. This improves the chances of data-recoverability on crashes or on filesystem corruption. Add a kill-switch toggle that erases all important files on the mac on 5 unsuccessful login attempts, and enables surveillance mode permanently using different tracking software (prey-project for example).
Could you expand a little or point me at some resources, that would be great thanks :-)
But here are opensource solutions:
http://musicbrainz.org/doc/Fingerprinting
Something like that would be ideal if a system could be implemented that was reliable enough, there's always going to be some degree of false positive/negative but it would be cool otherwise.
Well, OK, a mugger might, but would they recognize little fob thing as something of import that must be close to the computer?
Or, you know, just fence it.