I can think of a couple of options.

The first and simplest is to use something like a Boss Loop station pedal.
REcord your strings on a computer and then transfer them to the loop station plugged into the house PA. When you want the sounds you turn the pedal on when you want shot of them turn the pedal off.

Another possibility it to use a Multitrack recorder like a Korg D1200. You record the bits of orchestration onto the D1200 which you feed into the PA and turn the D1200 on and off using a foot switch.