This is currently not on our roadmap. You can retrieve this value by making a call through the Graph API. If this is needed for your scenarios, please continue voting and we will review at a later date.Kevin Ortman commented
Why allow UserName to be a first class IDP citizen without providing it's value in the JWT claim?
I've reluctantly reverted to EMail to avoid the extra hop required to extract that claim.
Can you share with us the type of test environments in these resource groups that you would like to be under DevTest Lab policies?
DevTest Labs now supports integration with Azure Shared Image Gallery (SIG).
For the scenario you outlined above,
You can extract the images for the VMs you would like to move to a lab
Upload these images to the shared image gallery
Attach the shared image gallery to your lab
Create lab VMs from these images
Upload images to a SIG: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/shared-images-portal
Attach a SIG to a lab: https://docs.microsoft.com/en-us/azure/lab-services/configure-shared-image-gallery
Create a lab VM from a SIG image: https://docs.microsoft.com/en-us/azure/lab-services/add-vm-use-shared-image