Unless I am mistaken (Which is certainly a possibility!), I don't believe you can suppress this message from the server side, as it is a setting on the browser client itself.
This is just a guess at a possible solution, as I did not have time to test it, but have you tried making all of the links on your page http links, and using a controller to redirect the client to https links as needed.
“Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning.” - Rich Cook