How to propose rescheduling a meeting sent by someone else in Outlook for Mac 2011?
So, it looks like Outlook for Mac 2011 is incapable of proposing updates for meetings sent by other people (or, for meetings sent by Outlook for Windows users), regardless of standard permissions. 

Looks like this is another missing feature alongside voting buttons and easy calendar sharing requests. In Outlook for Windows you can propose updates that the organiser can accept or reject; in Outlook for Mac there seems to be nothing like this.



This is based on the fact there are 'Send update' options seemingly only for meetings that you are the organiser of:



It looks like the only possibility is to decline the meeting, then send your own - or, if that's not an option (which will be true any time it's not appropriate to just become the new meeting organiser), you need to email the organiser (e.g. in a 'Decline' comment) and ask them to do it for you. 

A bit rubbish because, if they're a Windows user, they'll think you're a fool who can't find the 'Propose new time' button, and they'll think you're wasting their time asking them to do simple tasks for you...