The issue with salary sacrifice is your contract of employment. If you get an increased pension contribution instead of a pay rise, you will have to get a new contract to reflect this. The bigger the organisation you are working in, the less likely they are going to do this as they won't want al la carte employment contracts.
As for indicating you intend to retire early, I wouldn't care about that. If you are in your late 50's/ early 60's, they may want to get rid of you anyway. They don't own you and if you want to be in a position to make life choices earlier than 65, that's up to you and not them.
Steven
http://www.bluewaterfp.ie (www.bluewaterfp.ie)