This is somewhat related to the discussion of classical auxiliary verb ふ, mentioned in the answer to snailboat's question, What is the わ in 忌まわしい and 嘆かわしい?.
Another apparent iterative / repetitive ending that I've bumped into is る, attaching not to the 未然形【みぜんけい】 but instead to the old 終止形【しゅうしけい】. This suffix shows up much less often. Examples that come to mind are まく・まくる and むく・むくる, but I think there may be a few more that escape me at the moment. According to Shogakukan's Kokugo Dai Jiten dictionary, this form appears to have evolved from the classical 連体形【れんたいけい】, formed by adding る onto the 終止形【しゅうしけい】 for 二段{にだん} and 一段{いちだん} verbs (both 下{しも} and 上{かみ}). (I almost listed あく・あくる above, but I realized that あくる here is still restricted to 連体-only usage.)
Does anyone have any information on the process whereby this 連体形 form evolved into an apparent iterative/repetitive? Alternatively, does anyone have any explicit explanation of this formation and its semantic uses?