what steps did you do to get it working with DRM content on your rooted phone? The Shield can also be rooted. Apps can be sideloaded over ADB. I would like to test this because I would love to have less cables and a better picture quality for hyperion without noise/flickering, especially in dark scenes!
Mine worked right away. You can use the app DRM Info (available both on Mobile and Android TV) to check Widevine security level. My phone and Shield report Widevine L1, however when I open Netflix on the phone it reports using L3 (Playback Specification tab). L1 is the most secure version that allows high-quality playback, L3 allows the screen grabber to work but only at lower resolutions.
I feel like this is a pretty hard limit concerning all digital grabbing methods. Perhaps it would be possible to 'duplicate' the Shield's output at different resolutions (and thus Widevine certifications)?