git's core.SharedRepository is supported by git-annex, but only with the group/all/world/everybody settings. core.SharedRepository=0644 etc is not supported.
There's no insormountable reason why not, Joey just hates umask mode math stuff and nobody has sent a patch. Note that Annex.Content.freezeContent should remove the write bit from files, no matter what.
I don't think it would be that hard to implement, I count 6 places, all in Annex.Perms that don't handle UmaskShared.
For now I've made all of them display a warning.
(Note that your warning patch to datalad is probably buggy, since core.sharedrepository=1 and =2 are supported by git-annex. (As legacy alternatives to "group" and "everybody".))
I've implemented this today!
(Did notice a bug, ?wrong modes for some lock files withcoresharedrepository)