You can do this but the only way I can think of doing it at the moment is to modify part of the core.  Each of the skin files includes skin.vb as the code behind.  I have a routine that I use that is used to find the url for the site and the get all of the files in an image folder for javascript to display.  The code could be easily adapted so it would just find the url to the website and then add /Default.aspx to pull up the home page.  If you think you would be comfortable with this type of modification then let me know and I can post the code for it.  I may be able to modify it so you would just have to copy the code and add something to the skin.  I don't know if it would be easier to write as a control but for now this is the only idea I have.  I have used it on both parent and child portals and haven't had it fail yet.  You can post back here if you are interested and I will post it. 

Update...

I posted my skin to a remote server and lo and behold the links worked!  Skins are always stored in a folder at a predictable number of levels down from the site root... so a relative link works.  This gets around having to "hard" link.  The solutiuon from my earlier post...

The solution is this:  I put  a relative URL  into the href that targets the default page 4 folders up from the skin folder.  That hits the default.aspx "home" page for that portal.  href="../../../../default.aspx"