Good thinking Piglet..
If I may, I'd like to make a couple of ammendments to your suggestions.
(a) Include the use of Crossbows in General Archery. This way it only leaves Blackpowder requiring a slot.

(b)
Maybe something like this;

Sept1 - Oct 30...General Archery
Sept ? Oct ?......Blackpowder (non exclusive)
This way perhaps nobody will feel they drew the short end. Hope so anyway.