If anyone gets it to work it would be nice if they would share their configuration and method. Also U-boot version.
If there is a U-Boot variable to switch the initial video output slot/device then yes .

The only way I could get my X5000 to boot MorphOS with a Radeon HD Southern Islands card in the primary PCIe slot was by using the virtual monitor driver . For some reason I could not get the VNC properly configured to work though . I know that it booted because I could hear the MorphOS boot jingle.