[exim] dnsbl rotate/random?

I only have about 7 entries, most of which are sorbs entries with
=127.0.0.x entries..

I was wondering if there was someway to get exim to "round-robin" or
"random" or "rotate" through a list of dnsbls..

We recently got hit with some big mailbombs for a few days (exim was
just fine) but b/c we had spamhaus as our first hit; they said we were
over our 'quota' and said we needed to pay to continue to use them.

We did b/c it's a great service, and got a reasonable discount as a school..

But I was just thinking if there was anyway to do this..

Thoughts/Opinions/Suggestions ..