AFAIK, the SM-5 uses and electret element, and the Yaesu is designed to work with a dynamic mic. Check the pin-outs of the 890's mic connector. You'll need to figure out a way to connect the AF and PPT/ground lines, as well as provide power to the SM-5's mic element.
According to this manual, the AF and power travel on the same pin, pin 1. PTT is enabled between pins 5 and 6.
According to this manual, pins 5 and 6 enable PTT, but AF travels on pin 8 and there's no power coming from the rig at all.
So in order to use the SM-5 with the 890, you'll need to devise a circuit to reroute the AF and provide power.
The new signature restriction is bollocks.