Quoting Didier Kryn (kryn@???):

> Busybox, as from version 1.24.1, has a netlink listener meant to
> fork mdev (or any other app) on uevents. This is meant precisely to
> emulate hotplug behaviour in case it is disabled in kernel. The
> applet is named "uevent" . https://www.busybox.net/

There are other reasonable examples, too, such as Neale Pickett's:

Setting up mdev as a hotplug listener

The mdev utility of busybox can replace most of what udev does. You just
need to have the kernel run mdev as the hotplug userspace thingy.

Recent precompiled kernels have removed support for
/proc/sys/kernel/hotplug, so it's necessary to run a userspace program
to get netlink events. You can use the one I wrote[link] if you like.

You'll need to configure mdev to set up file permissions that work for
you. A reasonable set of defaults is available as another part of my
AUR.[link]

Since X11 wants udevd for something or other, you'll also need to tell
it to use whatever the old method is. I don't quite understand what they
do, but the files you need are in my runit-desktop AUR.[link]

https://woozle.org/neale/papers/arch-runit.html

Direct link to his uevent/netlink-handler program:
https://woozle.org/neale/g.cgi/aur/runit-init/tree/src/hurtplurg.c


> Nathanael Copa has considered transforming Mdev so as to read
> the netlink - together with other improvements. This has some
> advantages, such as not needing dirty tricks to serialize the
> processing of uevents. The advantage of keeping Mdev as it is now is
> that it is not constantly running (and consuming ressources) while
> it is rarely working except at startup; this maybe important on tiny
> systems.

Quite so. Reading netlink for kernel uevents seems like a solution for
a problem I happily don't have.