User-mode Linux can be ported to other operating systems that have the necessary functionality. The ability to virtualize Linux system calls by intercepting and nullifying them in the host kernel is essential. An OS that can't do that can't run this port. Also needed is the ability to map pages into an address space in multiple places. Without this, virtual memory emulation can't be done. If those two items are available on a particular OS, then this port can probably run on it.
It would be convenient to have the equivalent of CLONE_FILES, which allows processes to share a file descriptor table without sharing address spaces. This is used to open a file descriptor in one process and have it be available automatically in all the other processes in the kernel. An example of this is the file descriptors that are used by the block device driver. They are opened when the corresponding block device is opened, usually in the context of a mount process. After that, any process that performs operations on that filesystem is going to need to be able to access the file descriptor. Without CLONE_FILES, there will need to be some mechanism to keep track of what file descriptors are open in what processes.
Beyond those, this port makes heavy use of standard Unix interfaces. So ports to other Unixes will be significantly easier than ports to non-Unixes. However, those interfaces have equivalents on most other operating systems, so non-Unix ports are possible.