Forcing use of SSL connections in web dependent applications
A preliminary before getting to a solution: I think you would be unhappy with a solution which tried https with every site.  For sites which do not respond to https there would an unacceptable delay waiting for some timeout period before trying again with http.   

My solution for you is GlimmerBlocker.  This a proxy server running on your Mac.  As a result all web applications connect via GlimmerBlocker.  Though frequently used as an ad blocker, it has much more functionality including rewriting web requests or returning a redirect to your browser.

Here is a description of the Request and Response Flow inside GlimmerBlocker including how URLs can be modified.  Pretty sparse description, but can you started.

The first post in this blog has a more detailed recipe for converting http requests to https.  This can be applied to specific web sites which you know do support https as well as http.

Using GlimmerBlocker beyond what is out of the box is not for the fainthearted, but it has the power to do almost everything conceivable to web requests.