Have you tried customizing org-agenda-format-date with a function? Here's the doc string:
"Format string for displaying dates in the agenda.
Used by the daily/weekly agenda and by the timeline. This should be
a format string understood by `format-time-string', or a function returning
the formatted date as a string. The function must take a single argument,
a calendar-style date list like (month day year)."
If you're mostly concerned about displaying dates, rather than reading them in, that might do the trick.
-- Bill