To provide a brief answer to my own question, providing 'contacts list' functionality on the same page as emails in Roundcube requires an overhaul of the core logic (unless a replacement addressbook is written). The challenge is simply illustrated if you try and place <roundcube:object name='addresslist'> on the 'mail' template. Basic issues seem to be:
(populating a template with a list of messages) and task='addressbook' (providing a list of contacts) so that would need to be engineered out to allow use of existing 'addressbook' and 'mail' code on the same page.
program/steps/addressbook have the same name (e.g. list.php) with other code doing an include of program/steps/$task/list.php so the name clash would need files (and their references) renaming.
'addressbook' page assumes the same command 'list' means 'list contacts'. As far as I can tell this would need re-structuring to use different command names for the two functions for the lists to coexist on the same page (I am re-writing the code to use 'listcontacts' instead of 'list' for the contacts list command.)
Ian
List info: http://lists.roundcube.net/dev/ BT/aba52c80