Those "resonators" are cats. There's a section of cats in the headers, and a section that you see in the OP's first post before the axle-back section. No sensors connect pre or post "secondary" cat section on other BMWs (or the M) for that matter.
See Realoem. Here's
a link.
Now whether the car could pass a tailpipe smog check without the "secondary" cats is another question. As for an OBDII readiness check, since there are no O2 sensors around the "secondary" cats, it's not going to affect that. The only issue would be passing a visual test.
In the US in CA, the tailpipe test is to be replaced in 2013 by an ODBII readiness check, so the rules have changed. Things changed with
AB229, section 44012. Aside from the issues with the visual check, removing these may not pose the problem they would have at one time since this section isn't monitored by the ECU.