Problems with user-level security

Maybe it's just me, but I think pg_hba.conf needs a user column in it.

On the one hand, I need

local sameuser password

because I have untrusted users who should only have access to their own
databases.

On the other hand, I do a nightly pg_dumpall as part of the backups so that
I don't have to dump the actual database working areas. For that to work, I
need to say

local all password

so that my backup script will work.

What I *really* want is for an extra column to be added to pg_hba.conf to
specify the user of interest. Like this:

all local sameuser password
backups local all password
pgsql local all password

Does this make any sense? Is there some way to achieve this I am missing?
The only way I can do backups at the moment is introduce a race condition
so that anyone can connect to any database they like at certain times of
day. Not good.