With the following code flyspell at least shows such misspellings. The list is to be extended.
(defvar flyspell-wrong-one-letters '("i"))
(defadvice flyspell-word (before one-letter activate)
"Check one-letter words"
(when (and
(if following (looking-at "\<[[:alpha:]]\>")
(looking-back "\<[[:alpha:]]\>"))
(member (match-string 0) flyspell-wrong-one-letters))
(setq known-misspelling t)))
I just looked into flyspell not into ispell yet. I know that this is not really a solution for you... Sorry.
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