Manual anti-wraparound vacuums

In another thread, Alvaro quoted from the manual:
>     If for some reason autovacuum fails to clear old XIDs from a table, the
>     system will begin to emit warning messages like this when the database's oldest
>     XIDs reach ten million transactions from the wraparound point:

>     WARNING:  database "mydb" must be vacuumed within 177009986 transactions
>     HINT:  To avoid a database shutdown, execute a database-wide VACUUM in "mydb".

>     (A manual VACUUM should fix the problem, as suggested by the hint; but
>     note that the VACUUM must be performed by a superuser, else it will fail to
>     process system catalogs and thus not be able to advance the database's
>     datfrozenxid.)

It occurs to me to wonder how this scenario will interact with the
recent changes to let VACUUM skip pages.  AFAIR there is not a way for a
manual VACUUM to set the anti-wraparound mode, is there?  Perhaps manual
VACUUMs should not have the page skip behavior at all?
        regards, tom lane