Portage has a check (from bug 378869) which causes it to bail out if it needs to write to a readonly filesystem. I would be useful to have a feature to remount such filesystems as writable on-demand.
Created attachment 608556 [details, diff]
quick patch for testing purposes, automatically remount rw
You'll have to set DONT_MOUNT_BOOT=1 in order to prevent mount-boot.eclass from calling die in pkg_pretend.
Typically /boot is simply not mounted, and the mountpoint is writable, so perhaps the code for bug 378869 should be extended to check for unmounted filesystems rather than just readonly filesystems.