Yes, brume uboot can load file from ext4 filesystem, so if you put anther openwrt kernel image and rootfs in the ubuntu partition, it should work too. The os to load is configured by uboot envionment variables.
a nice write-up or new script utility to point to different boot locations and set the proper os.
for example if the user
wanted to boot gli-openwrt from partion1 of micro sd card.
brume_install --os openwrt --location microsd1 if=gli3.x.gz.tar
os meaning gl-opnewrt, ubuntu, clean openwrt, arch linuxk etc
and location being
microsdX= /dev/mmcblk1pX
usbstorageX = /dev/sdaX
emmcdata = /dev/mmcblk0p3