mute / unmute instrument via plugin?

Yes, you can set mute property for channels of this instrument — that would be the same as what the mixer does. This example shows how to iterate over these channels. If you need to do it only for a particular part or instrument you can find the relevant part for a given element with something like element.staff.part or just by searching for it in the list of all parts of a score, like shown in the example mentioned above.

sammik,

You're ahead of me! I was going to suggest the addition of Mute buttons to your scordatura plugin.

Do I assume correctly that the plugin would mute the scordatura track by default?

scorster