'login_url' filter issue with 'home_url()'

Hi

I have function making use of the 'login_url' filter, to use a custom user login page for sites on my network. In it I return a new login URL that I generate using the 'home_url()' function, since I determined that to be the best function to use to return the correctly mapped domain for a given site.

It doesn't work, however. My 'login_url' filter function works, since if I set it manually to another page on the site, or to google.com or something it works fine. It seems that I cannot properly use 'home_url()' (or 'site_url()' for that matter) on mapped domains when using Mercator. The function works fine for subdomains on my main network domain, just not for mapped domains.

Any ideas? Thanks.