The JACK folks and I have agreed on a little specification for device reservation that allows clean hand-over of audio device access from PulseAudio to JACK and back. The specification is generic enough to allow locking/hand-over of other device types as well, not just audio cards. So, in case someone needs to implement a similar kind of locking/handover for any kind of resource here's some prior art you can base your work on. Given that HAL is supposed to go away pretty soon this might be an option for a replacement for HAL's current device locking. The logic is as simple as it can get. Whoever owns a certain service name on the D-Bus session bus owns the device access. For further details, read the spec.
There's even a reference implementation available, which both JACK2 and PulseAudio have now integrated.
Also known as PAX SOUND SERVERIS.