watching dynamic device attachment?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I have an application where I want to watch USB devices come and go.
I've written code basd on usb(4), and it works - but the devices in
question are disks, which show up as, for example,

umass0: using SCSI over Bulk-Only
scsibus1 at umass0: 2 targets, 1 lun per target
sd0 at scsibus1 target 0 lun 0: <Generic, External, 2.10> disk fixed

So, I'm wondering if there's some way to watch devices come and go
beyond what /dev/usb gives me (which stops with the umass attachment).
Even some way to inspect the current device tree would help; I can
watch umass attach and then query the tree to see what's underneath it.
I have a fuzzy memory of seeing something that looked like device-tree
data, but the memory's too fuzzy to be of much use here.  kern.drivers
appears to be part of what I'd want, but only part; I'd much rather not
have to scrape dmesg output. :/

The machine in question is currently at 4.0.1.  If this is possible
with a more recent version but not with 4.0.1, I might be able to talk
its admins into switching, but I suspect they'd rather not; it _is_ a
production machine.

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