If an HTTPS resource is available, consider links to the non-SSL version to be broken.
Many sites support HTTP and HTTPS as part of a transition to a more secure Web presence. To help ease such a transition, Broken Link Checker could also inexpensively check to see if a resource accessed over HTTP is also available over SSL. If no SSL/TLS-secured response is returned, future resources at the same domain do not need to try SSL, since a pretty safe assumption can be made that the particular domain checked does not support it. However, if it does, then it would be smart to suggest that the HTTP version of the resource is "broken" because an HTTPS version is available.
Broken Link Checker could suggest a very simple fix: use server-relative URLs in the form of "//example.com/path/to/resource.html" instead of URLs that are hard-coded with HTTP schemes.
This functionality would also implicitly address ideas such as https://whiteshadow.uservoice.com/forums/58400-broken-link-checker/suggestions/2709569-problems-with-https-links