How are you going to know to set that? With some sort of process running on the server or the sending application?
Then now are you going to know if a server has been upgraded or not before / when a load balancing decision is made?
I was thinking something more along the lines of different Data Groups and Pools.
Data Groups to differentiate the upgraded and non-upgraded requesting servers (so that no other mechanism would be necessary to determine if a server had or had not been upgraded) and pools to separate the upgraded and non-upgraded (otherwise you would not be able to keep the load equal on a group of servers).