Triggers, triggers all the way I'm afraid...
I think it fails into category of "junction objects" (between User and Opportunity) and docs say email alerts aren't supported on junction objects.
Workflows don't work on
OpportunityTeamMember (the option is both inaccessible from GUI as well as forced creation of OpportunityTeamMember.workflow file fails in Eclipse).
<?xml version="1.0" encoding="UTF-8"?>
<description>Team Modified Notification</description>
Save error: OpportunityTeamMember.Team Modified : Cannot create a workflow rule on entity:OpportunityTeamMember
Save error: OpportunityTeamMember.Team_Modified : Cannot create a workflow action on entity:OpportunityTeamMember
Same story with AccountContactRole, AccountPartner, OpportunityContactRole etc. - several objects where we can't add fields, customize them (well, except the Role picklist but you get the idea)...
Bonus - looks like mailing Opp Team members is clunky in general ;)