Milter-greylist

I would like to compare the effectiveness of two DNSRBLs hosted by our
academic network provider.

The only way I can think of to do this would be to let milter-greylist
lookup each DNSRBL like this:

racl "PBL"   <just log the result>
racl "NewBL" greylist dnsrbl "NewBL"    delay 58m
racl "PBL"   greylist dnsrbl "PBL"      delay 58m

I would then count the hits for each DNSRBL in my milter-greylist log

Is there a way to achieve the <just log the result> bit, or even a
completely different way to do this?

Thanks

Mark