What mechanisms do you use to notify third parties of future schema changes? How do you notify third parties of future SQL Server schema changes other than through external documentation? 

I've started implementing a deprecation warning extended property on objects that will be going away in the future but wanted to see if anyone had success through another approach.
We always strived to make schema changes backward compatible. There are probably situations where this isn't possible, and obviously situations where they want to know so they can take advantage of new features, but we were able to do this consistently without breaking anything. I wrote a series of tips on mssqltips.com about this (look under "Database Development" here).

Extended properties aren't a very useful mechanism for this, IMHO. Since nobody is required to ever look at these, and since many tools ignore them completely, you may as well write out sticky notes and paste them on their monitors. How you would notify them in specific cases I guess depends on what impact the change will have on them and whether or not there are other ways you can implement the change to reduce or eliminate that impact.