The tint of a slot is part of its setup pose. As such, providing that method is something we'd be a bit reluctant about. However, you can record the slot tints right before calling the existing method, then manually re-applying the tints.

You could also write your own code to reset the slots to a partial setup pose.

I see you've already brought this up before while you were using Unity, and do have the extension code.:
SetSlotsToSetupPose

If you go that similar route for C, you would do the same.
Loop through all the slots.
Then do what slot_setToSetupPose does, but without resetting the color.