As posted in the chatroom, L4Linux kernels are already available with a user-mode patch for Linux. They used to have a corresponding L4Android kernel years ago but without drivers compatible with the L4Re microkernel, I'd imagine it was abandoned.
The biggest problem with porting to microkernel architectures is that the driver support has to be able to run in user-mode to be able to operate. On hybrid kernels like Windows or Linux, the drivers run in supervisor mode and can see anything the operating system can see and write to anything that the operating system can write to. Worst of all, a compromised driver set can operate with impunity and without any security enforcement over it, can compromise the entire rest of the operating system and anything running under it!
The main reason I have been reluctant to install GrapheneOS on my Pixel 7a is that without a microkernel-compatible driver set and a microkernel architecture to match, Google's own drivers being used on a Pixel phone could compromise the entire phone! I don't know a way to improve the security without totally reverse-engineering the Pixel drivers from the original Google drivers for Android on one hand, or finding another phone with open-source drivers on the other hand, there is really no way to enforce security on drivers at all!
If wifi or modem drivers are accessible to other drivers independently of interprocess communication (IPC) then all hope of a secure phone is looking quite lost! Without IPC enforcement, there would be no way to prevent spyware-embedded drivers from doing their thing undetected forever!