Mksquashfs ownership

I've been playing around with trying to resolve file ownership in the squashfs partition at build time. The default build system in include/ uses the -root-owned flag which allows a non-privileged user to create root-owned files in the image.

define Image/mkfs/squashfs
        $(STAGING_DIR_HOST)/bin/mksquashfs4 $(call mkfs_target_dir,$(1)) $@ \
                -nopad -noappend -root-owned \
                -comp $(SQUASHFSCOMP) $(SQUASHFSOPT) \
		-processors 1 \
                $(if $(SOURCE_DATE_EPOCH),-fixed-time $(SOURCE_DATE_EPOCH))

I had hoped that the pseudo-definition/file feature could also be used to have specific overrides of ownership:group.

-SQUASHFSOPT += -p '/dev d 755 0 0' -p '/dev/console c 600 0 0 5 1'
+SQUASHFSOPT += -p '/dev d 755 0 0' -p '/dev/console c 600 0 0 5 1' -p '/test-file f 600 50 50 echo test-file contents'

Unfortunately it seems that the file, while generated, ends up being root:root for ownership.

Does anyone know a "trick" that a non-privileged user can use, short of find everything and generating a pseudo-file that contains every file in the image?