Just to note that Plantuml already supports n.n autonumber format (see http://forum.plantuml.net/8714/autonumber-keywords-activate-keywords-differenct-automatically) e.g.
It also already supports user-specified numeric formats using Java class DecimalFormat and surrounding text. e.g. autonumber "<b>"
So one suggestion on how to implement the feature could be to extend the format options to include the multiple components (there may already be syntax for this?) - e.g. "autonumber "##0" "'.'##0" "'.'##0" "'.'##0" for a 4 numeric component formats separated by ".". And then add additional syntax to convert numeric to letters , roman numerals, etc. I appreciate this is probably quite a lot of coding, as I suspect the donkey work is done by Java class DecimalFormat which doesn't seem to support this, but it could just be a search and replace of a suitable symbol before passing to DecimalFormat - eg replace %a_to_z% with "a".."z" for 1..26 (use mod 26 if you want to get really fancy and support > 26 as multiple characters) before passing to DecimalFormat.