Quote Originally Posted by Ian Grant View Post
Copper toner/intensifier might work but the resulting negative is unstable.
I think copper intensifier is a completely different animal than copper toner. Copper intensifier appears to be mostly Copper II Bromide which works as a rehalogenating bleach. After bleaching one can intensify in Silver Nitrate, or some staining developer. There is nothing inherently unstable about the resulting negatives.

Assuming that the mysterious bottle indeed contains CuBr2 I would assume it still works and can be used.