Regarding kanji groups with few kanji in them: these groups can be attached to several smaller phonetic keys composing overall reading. So that instead of single rare keyword for group we'll work with multiple frequent keywords for individual syllables. Kanjis in group may be placed in context involving all those keywords (somehow ordered).
Example:
For reading オツ, keyword for オ is Oz, keyword for ツ is tsunami, so compound keyword is (Oz crushed by tsunami), story involving the only kanji in group 乙 (fishguts) is "Fishguts is all that remains from Oz-in-form-of-great-fish crashed by tsunami".
Example:
For reading キツ, keyword for キ is key, keyword for ツ is tsunami, so compound keyword is (very important key lost as a result of destruction brought by tsunami), story involving kanjis of the group 詰 (packed) and 喫 (consume) is "Monk of secret order has packed some secret keys entrusted to him by his order in a secret place, but subsequent tsunami consumed all the village, along with his pack of secret *keys*".
P.S. On index page some onyomis have a space in them (between letters), so link becomes incorrect 'cause of extra %20 in it. For example, link for ウオ is
http://kanji.koohii.com/onyomi/ongroup. … 0%E3%82%AA
(which doesn't work)
instead of
http://kanji.koohii.com/onyomi/ongroup. … 6%E3%82%AA
(which does)
But even though it works, section doesn't contain keywords (while index indicates that there's one). When I enter ウオ in lookup editbox, it tells me that group's not found. If I enter ウ オ (with space) it starts some _review_...