Okay, this seems to work for me to tag the RTP packets associated with a call setup by SIP:
In “IPTABLE PARAMETERS” free form field, enter the following:
-m helper --helper sip
From what little I turned up during searching for this, it appears that you need to tag the SIP flow first. Or maybe have the nf_conntrack_sip module loaded. It sure does not seem to be well documented.
But based on a sample size of two calls, it does seem to properly detect the RTP stream and classify it for QoS.