> I'd like to add a sysctl to get all the devices in the kernel.

It seems to be better to use a pseudo device rather than sysctl
to achieve this. Because...

> Why do I want this?  Well, I'm messing with USB (as usual :).  Right
> now I can get a message (by using the USB event mechanism) when
> a USB device is connected.  Say that I connect some speakers, I will
> get a message that, say, "uaudio2" has been connected.  But which
> audio device does this correspond to?  There is no way to find out
> except by getting the device tree.

If you use sysctl, you have to use busy wait to detect a change
of device configuration.
If there is a pseudo device which reports attaching/detaching event
on its read(2) entry, you can use standard read(2)/poll/(2)/select(2) 
interface to wait the change.
--
soda