[exim] different user than EXIM_USER for authenticator?

hi all,

from the spec:

"In some operating systems, PAM authentication can be done only from a
processrunning as root. Since Exim is running as the Exim user when receiving
messages, this means that PAM cannot be used directly in those systems"

leads me to suspect that authenticators CANNOT be config'd to use a *different*
user.

specifically, i'm hoping to:

    run exim as exim:exim
    run dovecot as dovecot:dovecot

have both access a flat passwd-file, "my_pwd_file" that has myauth:myauth
ownership.

in dovecot i can specify authenticating AS "user=myauth", but in exim -- iiuc
-- there's no 'option' to add to:

        begin authenticators
        plain:
            driver                  = plaintext
            public_name             = PLAIN
            server_condition        = ${if crypteq{$3}\
        {${extract{1}{:}{${lookup{2}lsearch{MY_SEC_DIR/my_pwd_file}{$value}}
        }}\ |{yes}{no}}
            server_set_id           = $2
        ...

is there an option i've missed, or -- per the doc/spec snippet above -- is it
not currently possible?

thx,

richard