I've tested it on an X200 + GNU Boot 0.1 RC3 + GRUB + high resolution + QWERTY and the trisquel-netinst_11.0.1_amd64.iso image and trisquel-netinst_10.0.1_amd64.iso.

The "Search ISOLINUX menu (USB) [u]" menu entry doen't find anything in both cases.

GRUB find the USB key and I can see it:

grub> ls
[...] (usb2) (usb2,msdos3) (usb2,msdos2)

If I try to load it by hand, with Trisquel 11 it fails, and we have the same issue with syslinux_source:

grub> set root=usb2
grub> ls /
[...] isolinux.cfg [...]
grub> syslinux_configfile /isolinux.cfg
error: bitmap file '/#00000000' is of unsupported format.

With Trisquel 10 it loads the Trisquel logo (splash.png) with both syslinux_source and syslinux_configfile but then we end up with a grub prompt and that's all:

grub>                         [ splash.png 8.57KiB 100% 30.69B/s ]

In all cases 'ESC' gives me back the GNU Boot menu (but without the background picture in the case of Trisquel 11 and with the Trisquel logo in case of Trisquel 10), so I can't boot Trisquel with syslinux.

Most distributions also provide GRUB configuration and syslinux works fine with SeaBIOS in text mode, but this is workarounds and it complicates things a lot for users and also for GNU Boot contributors as well in case we want to have a manual that is simple and consistent, which is a bit the point of GNU Boot anyway.

Another thing is that some distributions do work with syslinux. For instance with parabola-i686-systemd-cli-2024.02-netinstall.iso I can simply boot from the "Search ISOLINUX menu (USB) [u]" menu entry. In there memtest86+ even works with GRUB high resolution booting from "Search ISOLINUX menu (USB) [u]" and then selecting memtest86+.

A solution here would be to fix it and add an automatic tests based on the Trisquel 10 and 11 installers. We'd need an easy way  to modify somehow these installers to shut down at some point.