What can I use to flavor savory chicken stuffing instead of onion?
I use dehydrated onion soup mixture as a substitute to many preparations calling for onions. I have never stuffed chicken with it, but I would do it without any doubt. 

The trick is to get to the appropriate moisture and salt point. I use both water and some other sweet liquid (7up, sprite, coke...) to rehydrate and reduce the slight salty taste of the dehydrated onion. 

For your case, besides the sweetening, the soft drink will as well add some carbonic which may also help in aerating a bit the stuffing.