The first thing I do when purchasing a camera/camcorder is place a UV protective lens filter and leave it in. There is a little switch embedded on the lens filter threading of the HDR-SR11, just inside the flash, which must tell the camcorder that a conversion lens is installed. When you screw in the filter, a no flash icon (flash icon with a diagonal line through it) appears just below the face detection icons and the "Flash Mode" on the menu is inactivated (grayed out).
I personally consider this a design flaw. The only workaround I know is either remove the filter in low light situations or leave the filter off completely.
Anybody have any other ideas?

I concur with your views. I am thinking of cutting off the sensor tab, seems to be only plastic.

Yep, I use the Sony HVL-HFL1 Flash/Video Light.

What I do is just unscrew the filter slowly until the plastic switch is released again (you can see the flash icon changes) to activate the flash mode. There is at least a complete turn left before the lens filter is removed, so it is still stable at that point when the switch is disengaged, no need to worry about the lens falling off. Just screw it in again when you don't need flash light anymore.