wp-admin redirect to wp-login broken with non-default SSL host

Of course, I’ve just realised that my workaround doesn’t work if you request the secure /wp-admin/ while not logged in, because that still sends you to the www URL for the login page, and thus breaks. This needs fixing in the plugin.

Workaround: I added a filter on ‘wp_redirect’ that just does a str_replace for ‘https://www.example.com’ to ‘https://secure.example.com’. I did have a go at working out which function from WordPress HTTPS I should be calling, but the ones I tried didn’t quite do what I expected.

Okay, after further debugging, there’s a bug with the makeUrlHttps function. I haven’t tried to debug it fully because the function is huge and there’s no obvious trigger (for example, some scripts included on pages are broken, but some others work properly). I just have the following workaround inserted before the return statement in that function and everything is now working correctly (including the redirect to wp-login):
$string = str_replace('https://www.example.com', 'https://secure.example.com', $string);