I agree with you about the ability to use Maths Channels for Triggers (or allowing maths within the Trigger options, but using a Maths Channel is easier and reduces the risk of the Trigger and Maths Channel being designed subtly differently by mistake).
In you specific application this might be especially important if it's an intermittent issue, where huge volumes of data could otherwise be generated. We had a similar issue with a dual potentiometer, where maths triggering would have saved lots of time, for us the signal never exceeded or fell outside the min max bounds, it was the incongruence between the two that we needed to monitor/capture.
Is the signal you can see exceeding the normal logic levels of the CanBus? Then a simple rising or falling trigger might work?
Otherwise you might, with trial and error, ... and a few false positives, be able to set the pulse width window or advanced edge to do this? If you have a capture you can share, then it might be easier to see what options could be made to work or even be reliable?
+1 for Maths Channel Trigger to be added to the Feature Request list
PS I had some issues using triggers last week, for an old parasitic drain test template (used it for years with no issues, probably over 500 times) - triggering on door locks locking, 16amps showing on screen, reduced trigger from 10amps to 5,3,1 (simple rising edge) but still refused to trigger! We've had lots of issues with this laptop (Win7x64) using Pico (image preview etc) so we're going to do a deep uninstall, re-install and retest when we get a moment, but still at a loss as to why the trigger did not work? Just thought I'd add in case anyone else has had an issue using an old template or with triggers in general?