Does AWS ever `vacuumlo` PostgreSQL RDS instances?
Probably not, because blindly executing vacuumlo on random databases would be foolish.

Unlike vacuum that is pretty much unavoidable on a normal live database, vacuumlo fixes an oversight, a situation that should not arise in the first place, since applications should unlink the large objects that they no longer use, at about the same time that they delete the references to these large objects.

In a database with a large number of large objects that are properly handled, vacuumlo would burn CPU cycles and I/O at each invocation, just to compute every single time that there is nothing to remove.

Moreover, there are two assumptions that vacuumlo must do that are debatable:  


the fact that any column of type oid should be taken as a candidate to a large object reference. oid are used for other purposes, especially if you want to refer to objects in the catalog.
the fact that, if a large object doesn't have it oid refered in some table in the same database , it should be removed. If the reference is in another database, or in a file, or in the same database but embedded in a text column, that logic doesn't work and is in fact dangerous.