Targets I maintain / develop (bcm47xx, bcm53xx and bcm4908) don't require OpenWrt-custom sysupgrade images. It's a single image that can be used for both:
- Initial installation (from vendor custom firmware)
- OpenWrt's
sysupgrade.
So worry not - just use the available firmware image.
OpenWrt's sysupgrade on those targets can be used for both:
- Reverting to vendor firmware
- Upgrading OpenWrt
I believe / hope it's actually a simpler & easier solution. Get image and flash it. Don't worry about vendor vs. OpenWrt, factory vs. sysupgrade.