On the "Bitbake variables" page it says that SSTATE_MIRRORS cannot be changed. Since SSTATE_MIRRORS doesn't actually impact the build server and only the sstate fetch location, it shouldn't be restricted. This is especially true in the case of using toaster in a container versus a persistent build manager.
https://lists.yoctoproject.org/pipermail/toaster/2016-June/004887.html
The patch for this task has been merged upstream: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=be00eb29f3a66684bc9fadb278b5cf59947166fc http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=f433664fa042eb8a401fd50472813dc88fd80cde Hence marking this task as resolved-fixed.
Created attachment 3293 [details] SSTATE_MIRROR no longer a reserve variable
In attachment 3293 [details] you will see that SSTATE_MIRRORS is no longer a reserve variable. This was verified in master branch commit 627d01997fcf6a0581d88047735769ffb2592b82