I found a nice in-game addon called MailExpiry that logs all character mailboxes across all servers and warns me in-game when mail is about to expire. It would be nice to provide similar functionality in Roster, at least for one character, or perhaps guild-wide.

Roster has a facility to identify alts so one could use that to identify all mail associated with one account and present it in one page, sorted by expiration. One could also provide a guild-wide list of all email. This would make it easy to check before one logs in to WoW to jump into those characters with mail in danger of expiring.

Roster does not have the ability to tie alts with main characters

Ah, I saw the alt-text variable in the guild config and thought it was intended for that. Reading the fine print, I see it's just used to display a count on the main page.

So the ability to display per-account information isn't really there, and probably not doable unless one constrained the alt field to a special format (non-trivial).

That leaves the ability to provide a guild-wide mail expiration page as the next-best thing.