Installing OpenWrt x86 on GPT disk

How can I install OpenWrt x86 on a disk which is GPT formatted?

Looking at the installation guide here:- https://wiki.openwrt.org/inbox/doc/openwrt_x86

it looks as though the image is written to a device not a partition:-

dd if=/mnt/openwrt-x86-generic-combined-squashfs.img of=/dev/sda

so I presume it creates an MBR rather than GPT formatted disk...

Yep, currently there is no proper GPT disk support.

I can install it but can't figure out the Grub menuentry to boot it correctly...

I would argue that for initial installation it wouldn't make a lot of sense to build "GPT images", especially since you would still need to manually (re)create the secondary GPT header at the end of the disk, at which point it becomes no less effort than converting a disk from MBR to GPT.

(GPT support for sysupgrade would be rather nice, though.)

dd the image to the disk, (re)convert the disk to GPT using gdisk while it's still connected? Am I thinking too straightforward here?

I've never used gdisk before until now, and maybe I didn't use it correctly but after rebooting the system stopped with a GRUB statement - not a prompt. It didn't get as far as a GRUB menu.