Hi,
I have a BB-X15 at home still with the default image (kernel 4.9.??) on the eMMC. Regrettably, I've never tried running X on it (not even remote desktop).
I will test it tonight by plugging an HDMI cable into it..
I HAVE plugged an HDMI cable into our 572xEVM/BB-X15 at work a few months ago, with a "base" BeagleBoard.org LXQT image on SD Card for BB-X15 from BeagleBoard.org, and X worked fine on that. Note, that the 572x LCD touchscreen was attached when I tried this.
Please ping me if I forget to follow-up...