Problem solved, I should have looked in the modding forum. Anyway for anyone else who's stuck, what you do is not only make them playable in descr_strat but also insert a description into Rome - Total War\Data\Text\campaign_descriptions.txt. For example:

{IMPERIAL_CAMPAIGN_ARMENIA_TITLE}Armenians
{IMPERIAL_CAMPAIGN_ARMENIA_DESCR}Armenia is a nice place.

This seems to do the trick. Actually I did try it before starting this thread, but it didn't work because I spelt Armenia wrong - d'oh!