I mentioned this several months ago, around the time that the now() function was introduced, and was told that there is no capability for formatting the time values.
How about your [date (filename friendly)] field?
Mine outputs as yyyyMMddhhmmss, and the following expression:
mid([date (filename friendly)],0,4)-mid([date (filename friendly)],4,2)-mid([date (filename friendly)],6,2) mid([date (filename friendly)],9,2) mid([date (filename friendly)],11,2)
returns the string you're after.
You can either use it to create a calculated database field, and then use that in your rename rule, or incorporate the expression string directly into the rename rule itself.
If it's something you might use regularly, I'd recommend creating the library field for the sake of simplicity and safe keeping.
-marko.