I tried this as I have SSMS 2005 and SSMS 2008 R2 installed on the same machine.
opening SSMS 2005, I CAN connect to the 2008 R2 database.
When I check database options, it does not have an entry in compatibility level, though in object explorer it does report the server as 10.50.200
Obviously no intellisense, but it seems to work fine.
However the design view doesn't work as you state, but nothing to stop you using ALTER TABLE etc