[exim] Using verify_mode successfully?

Hi there,

I'm trying to use $verify_mode (exim 4.87) in the data expansion on a
redirect router, in essense:

testrouter:
    driver = redirect
    (conditions)
    data = ${if eq {}{$verify_mode} {:blackhole:}{:fail:} }
    allow_fail

I realise I could split the router out into a no_verify/verify_only
pair, but with 18 routers already it would be nice to be more concise.

Testing indicates that it's not expanding:

testrouter:
    driver = redirect
    verify_only
    data = :fail: V="$verify_mode"
    allow_fail

Fails with V="", which I wouldn't expect. Am I doing something horribly
wrong, or does this look like an issue? Is the variable only set during
precondition tests?

--
Dave