I love WinSCP and would like this feature. I recompiled OpenSSH to only allow group key exchange and had changed it to only allow SHA-256 but until the newest putty.. this was not supported.
Anyway.. if you have time one day, this could be a decent feature. I do realize that SHA-1 is probably good enough.. but I like the extra security.
Great job with WinSCP by the way.. everyone at my office swears by it to interact with our UNIX servers.