perhaps common enough for io.  definitely belongs in that entry if anywhere

I agree. I propose adding 一とたび as an "io" to the 一度 entry.
This can then be rejected.

99.9% sure this is just a typo (or very strange okurigana) for ひとたび.  88 real 
hits

http://www.sal.tohoku.ac.jp/englit/tohokueibun01/Proceedings56.pdf
http://records.viu.ca/~johnstoi/lucretius/lucretiusbookoneweb.htm

I couldn't find this anywhere, but compared the same passage of a Japanese 
and English translation I found of Lucretius's The Nature of the Universe, as 
follows, to deduce that it simply means "once":
"Just as dogs, with their noses, often find
the lair of some wild beast which roams the hills,
once they have found the right tracks on the path,
although the den was hidden in the leaves,"
「例えば,猟犬が一とたび道に,獲物の確か な足跡をさがし当てれば,山をさ迷うけだ
ものの,木の葉でかくれた巣を,往々鼻でかぎだす ように,」