[exim] Refuse all incoming connections if load too high?

I'm sure I must be missing something obvious but, er, it's not obvious
what.

I'm looking for a way to refuse (4xx) _all_ incoming connections if the
load average is too high. I'm aware of smtp_load_reserve and
smtp_reserve_hosts, but I want something that applies even to the reserved
hosts. I had assumed that there would be a main configuration option for
this, but I can't see it. Have I missed something?

At the moment I'm thinking I will have to do this with smtp_acl_connect
and a condition test on $load_average, which I guess isn't exactly rocket
science. But it seems odd that, with all the other main config options in
this area, there isn't one for this? Heck, even sendmail has one!

TIA,
Richard (who's now waiting for someone to make him say "D'oh!")