why not just have the welcome part like you have it and then change all the rest to phrases? or do like vbulletin does for it Welcome, {1}.
Code:
<phrase 1="$bbuserinfo[username]">$vbphrase[welcome_x]</phrase>
or for that part make your own phrase and then have the part in admin where they can add the rest. I think its nice like it is only certain things I would go with phrases on.