Here is a revised sub rule, based on conversation with a dozen peeps last night at Mitch.
(1) A substitute should normally be of equal or lesser skill than the person being substituted for. (2) League players should normally be ineligible to sub for another team, but may be used in exceptional circumstances. The opposing team must approve any sub based on the foregoing two points. The Management Committee (or, on short notice, the game ref) will arbitrate if necessary.
If a sub is used the team using the sub will normally be eligible only for 2.5 points for a win (rather than 3 points), subject to the discretion of the Management Committee.