Anywhere. There are lots of examples on the topic ; running NetBSD driver code inside NetBSD's user-land is the simplest use-case (https://www.netbsd.org/docs/rump/sptut.html), but that code is portable without modifications to just about any context possible.

Check out Antti Kantee's The Design and Implementation of the Anykernel and Rump Kernels if you want details on the architecture.