calculating traveller/sheet loads for square sails

There might be a slight advantage in sail shape at narrow sheeting angles, but it's a mizzen so this isn't really relevant. Maybe when setting a riding sail dead flat. On the other hand, in traditional working craft the boomkin (bumkin, or outrigger earlier still) might be used for other purposes than just sheeting the jigger.

It still comes back to the mizzen transverse load, though, when determining what might end up coming down the mizzen sheet. And that can be estimated by RM * .5 * .5beam at mizzen chainplates, * the safety factor. (Skeene's 8th ed.) Cunliffe and Leather talk about unstayed bowsprits (no bobstays), basically pointing out they were excessively over-engineered for the compression loads and then thoroughly abused, and very rarely carried away. Rendering outriggers were literally lashed aside the sternpost when and if they were needed.

The traditional method of figuring out appropriate scantlings may be required here. Build something by guess, then start removing material until it breaks. Then fix it just stronger than it was before it broke.