So I found that by extracting the formatting pattern from getDateInstance() and getTimeInstance() it is possible to determine if the default time format is AM/PM or 24h, and if the default date order is MDY or YMD.
I can easily get input-field and input-time to adhere to this without any need to change any of the Action classes that process the input, since the Action classes just add 12h if the hour value is PM.
It's quite easy to change the display order of input-date too, but since the display order is determined by the order the tags appear in the HTML I would need to duplicate the year, month and day <select> tags in input-date/page.jsp.
Would this be considered OK?
I don't like code duplication, but the only alternative I can think of is to extract month, day and year <select>'s into 3 separate .jspf's, and then include them in different order depending on the Locales preferred order.
