Berk - interesting and thought-provoking work. You are of course right to make the point about ex post identification of moderators being usually suspicious if not down-right dodgy, so your disclosure is important. In your case, the logic behind the classification of the variable and the additional meta-regression results (together with the circumstances around the calculation of the synthetic effect sizes) do lend credence to your findings.
There are also a few broader points from your review for the research synthesis field about 1) attention to appropriate classification of interventions (programme design), which you make, and also 2) programme implementation, in this case through enforcement of the condition, matters. I would also add that 3) 'lumping' interventions (CCTs and UCTs) together in one review case has enabled some very useful comparative effectiveness analysis to be undertaken. Will look forward to reading the final product including those all-important findings on test scores.