Hi,
For instructions on how to get GNOME OS in a VM, see GNOME OS call for testing (+BuildStream workshop). It’s also possible to install on bare metal, although this isn’t the main focus. So far @valentindavid has been focusing on Pinebook as far as I know and documenting it here.
It’s very much possible to build your own image from a branch of the gnome-build-meta repo. To test an unmerged branch of mutter, you’d need a branch that updates the track: field of mutter.bst to the corresponding mutter.git branch name. Push and the CI will build it for you. When that completes, the green tick icon next to the CI pipeline will give the option of starting an x86_64 VM image build, which you can then download.
If your use case is testing a change that affects only one package, and you want to run it on real hardware, you will probably get quicker results patching and rebuilding the ‘mutter’ package provided by your distro. The way to do this depends on your distro – best to ask in the appropriate distro related forum