How to disclose in a review of an edited book that one was offered a chapter?
I would suggest that you do not need a disclaimer in this particular case: the bottom line is that you did not contribute to the book. If you had participated, it would be a clear-cut case… yours is more complicated, but the fact remains that none of the text of the book was written by you!

Another way to see it: a disclaimer would not raise distrust, but it would certainly add confusion (or at least, not add any clarity). You'd have to explain concisely your situation, stating the facts. You could come up with something like “The author of this review was asked to participate in the book, but had to decline due to time constraints”.

Finally, if you want to be sure to do the right thing: ask the journal editor! You then make it his call, since it's his journal. He's the best person to ask, really…