I suspect that this is a *really* bad idea. It's very very hard to get inplace updates right. We have several unfixed correctness bugs that are related to the use of inplace updates. I really don't think it's wise to add additional interfaces that can reach inplace updates unless there's really no other alternative (like not being able to assign an xid in VACUUM to be able to deal with anti-xid-wraparound-shutdown systems).
In this case, the alternative is an immediate doubling of the size of pg_class right after a restore/upgrade.