Is it possible to customize the build version number further than X.X.$Build.BuildNumber$? The configuration page says that the version format string can contain expressions, so I think the answer is yes, but there's no drop down to search expressions.
I would like to use a format similar to ABC.YYYYMMDD.$Build.BuildNumber$ as this is what we manually derive for our documentation, but as we implement Continua it would be better if the logs/build history matched. ABC would be plain text, while YYYMMDD is year/month/day.
Note that the expressions available here are limited as the version is applied at the start of the build. The Set Build Version Action allows you to modify the version during the build. You can also update the build version by writing Custom Log Messages from any Execution action that can write to standard output.
See Expression Objects for the (almost) full list of expressions available thoughout Continua CI. We don't have a list of which expressions are available for each field type, but that can be determined from the autocomplete dropdown.