iptables: rule to bypass NAT helper?

I know that it's possible to bypass connection tracking with the NOTRACK
target, but is it possible to just bypass a conntrack and/or NAT helper?

The scenario is this: I have a Linux-based firewall serving multiple
clients. At the moment is has 14 zones. There are several SIP-based VoIP
services in use, and unfortunately one is rather braindead; turn on the
SIP NAT helper and it stops working.

What I'd like to do, is to keep using the SIP conntrack/NAT helper, but
somehow let SIP packets from one particular subnet bypass the helper.

Is this possible?