The Monado OpenXR implementation provides a "QWERTY" driver for testing applications using the keyboard and mouse.

This is clearly documented on the Developing with Monado page, but I didn't have much luck when searching for "monado qwerty driver" or similar phrases.

To cut to the chase, start the monado-service with the following environment variables:

QWERTY_ENABLE=1 XRT_DEBUG_GUI=1 monado-service

Depending on the Monado version (at the time of writing this includes the version in Nixpkgs), you may need to use an older environment variable to enable the debug GUI:

QWERTY_ENABLE=1 OXR_DEBUG_GUI=1 monado-service

This should open a separate window that can be used to provide input to running OpenXR applications. You'll likely need to resize this so that it can sit along-side your OpenXR window.