Hotplug2 project is a trivial replacement of some of the UDev functionality in a tiny pack, intended for Linux early userspace: Init RAM FS and InitRD. The binary is (x86, dynamically linked against glibc) 15 KiB with -Os (v0.7).
Yet it includes ability to parse rules and execute actions accordingly. It basically takes the best from both worlds -- the flexibility of udev with the distributing events design of hotplug. It can be used to create devices or to just load modules (which is it's primary goal).
I have to re-emphasize that this is not an udev replacement for desktop boxes. This is udev replacement for specific cases, that is early userspace and possibly embedded devices. That's it.