How to receive notification of a new (disk) device being connected?

[TJ]

I'm currently finalising a patch that adds key-file support to 
cryptodisk::cryptomount and luks.

As well as being able to grub_device_iterate(hook,...) to search the 
existing devices for the key-file the code needs to wait for TIMEOUT 
seconds for additional (usually USB Mass Storage) devices to be 
connected and when they are search them for the key-file.

Is there an existing mechanism to receive such notifications, or a 
blocking call that returns if a device is connected or a timeout is hit?

Otherwise it looks like I'll have to track which devices are connected 
when the TIMEOUT loop is first entered, grub_sleep(), then 
grub_device_iterate() and see if the device list has any new entries.

Secondly: am I correct to think using grub_search_fs_file(keyfile_name, 
variable_name, ...) is the correct way to search for the key-file across 
all devices, or do I need to use a specific hint type to indicate all 
devices should be searched?