Bob,
the primary purpose of the rule requiring structural support when (structural) coamings are removed appears to me to protect owners from damaging their own boats. Being allowed to cut the coamings down flush with the deck (on an original DS1) gives you the definite competitive advantage of making it easier to hike out. There seems comparatively less competitive advantage in being allowed to further remove the remaining bits of the coamings - I simply can't believe that removing such modest remaining weight this close to the waterline has overwhelming effect. My view would be that this background should factor into how the class rules are defined. Mike is right when he compares that to the rule for the forward bulkhead.
The question then seems to boil down to, what kinds of solutions give you adequate structural support. The original DS1 has wood carlins into which the coamings are screwed. If someone were to replace these by a rectangular and stiff hollow profile, then I would think that could support the side deck just as well as the original coamings did. Ditto for replacing the carlins by somewhat beefier hardwood version that is well bonded to the deck. Struts, if spaced closely enough, would of course work, as would beefing up and increasing the number of plywood triangles (brackets) between hull and side deck. As would filling the gap with flotation and walling it off.
I don't see a problem with the intent of allowing other means of reinforcement, but unlike the bulkhead, the number of reasonable solutions is much larger, owing to the long unsupported span. This would be reflected in the wording of the rule. Here's a suggestion:
Wood coamings may be removed where the side decks are supported for substantially their whole length by a wall or series of struts connecting them to seats or flotation tanks or by strengthening of carlins and brackets, including additional brackets, under the side decks, sufficient to make up for the loss of structural support from removing the coamings.
You could also give a maximal allowed distance between struts and another, smaller one, for brackets, as well as minimal cross section for carlins. That would make measurement decisions easier for those solutions, but it might be overengineering the rule, because I see its main purpose in preventing people from damaging their own boats. Therefore, requiring them to do at least something should be enough to get them to think about the consequences of removing a structural element from their boat, without either having to limit the approaches or somehow verifying that it is 100% equivalent or better.