JPK:
Here is a photo of the intercepting sear angle on a typical Perkes-patent intercepting safety, in this case from my Scott .500 No.2. It was probably the most widely-used interceptor on 'vintage Brit' back-action sidelock DRs. I can't imagine for a second that the designers had in mind that it could operate as a 'auxillary trigger-sear' as suggested above. The angles are hopelessly wrong, being designed to 'catch and hold' rather than release.

If that doesn't convince you, check out the leverage on the trigger-sear compared with the intercepting-safety sear. The trigger sear has a leverage of 2:1 in favour, whereas the interceptor has a leverage of 3:1 against!

I have no doubt that a strong man with big fingers could possibly wrench a Perkes intercepting sear out of engagement with enough brute force, but with a high likelihood of something being bent or broken! I'm certainly not going to test the theory on any of my Brit doubles!

Regrettably, I don't own any high-grade bar-action sidelock DRs for comparison, a situation I'm always trying to rectify!