About ten years ago, there were a series of experiments (since gone nowhere), of allowing position players to pitch, say, one inning as a "reliever," particularly if the team were losing badly (the New York Yankees' Nick Swisher, the Pittsburgh Pirates' Josh Harrison).
"Normal" relievers loosen up in the bullpen before actually pitching. I can't see position players doing this.
My understanding is that once on the mound, a reliever gets eight pitches with which to warm up.* Is this all that the position player/reliever gets?
*An exception would be made if the previous pitcher was injured. Then the reliever would get all the time reasonably necessary.