I think the rules have some reasonably clear implications on this issue:

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2.6 Spirit of the Rule

2.6.1 In case of doubt, the intention of the rule makers, which is the spirit, shall take precedence over the letter of the rule.

2.6.2 The spirit of the rule includes, among other principles, the following considerations :

2.6.2.A Preserving general equality in overall performance between crafts of different make, accepting small variations, in order to garantee fair racing between designs of different make.

2.6.2.B Maximizing the freedom to optimize a design to personal preference and to improve the performance of a given crew and craft through refinement.

2.6.2.C The allowance to gently improve, by design, the handling and overal behaviour of a craft in small controlled steps which don't upset the balance in the class to the extend that the continued existance and growth of the class are no longer garanteed.
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It seems to me that if canted boards could be shown to be responsible for a substantial and systematic decrease in wetted area, they would

(a) fall outside the goal of "general equality in overall performance between crafts of different make",

(b) not be viewed as merely a matter of personal preference (which I take as being intended to stand in contrast with anything aimed specifically at an increase in the potential speed of the boat), and

(c) not be an example of measures designed to "gently improve, by design, the handling and overal behaviour of a craft in small controlled steps which don't upset the balance in the class to the extend that the continued existance and growth of the class are no longer garanteed"

Mark.