I ran into the same problem about a year ago. I found some information about modifying the ossssearchresults.aspx in the 12 hive (located in c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS) to use javascript to redirect to the custom results.aspx page. For example, see this page:
http://sharemypoint.wordpress.com/2008/02/11/redirecting-from-osssearchresultsaspx/
However, I didn't want to affect search functionality for the entire farm, just one particular site collection. I added a simple check to only redirect the search if the original search came from my particular site. The resulting javascript looks like this:
function getParam(name)
{
name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");
var regexS = "[\\?&]"+name+"=([^&#]*)";
var regex = new RegExp( regexS );
var results = regex.exec( window.location.href );
if( results == null ) return "";
else return results[1];
}
function redirectTo()
{
urlTmp=getParam("u");
if(urlTmp.indexOf("yourdomain.com%2Fsites%2Fpath_to_site")>0)
{
window.location="http://yourdomain.com/Search/Results.aspx?k="+getParam("k")+"&cs="+getParam("cs")+"&u="+getParam("u");
}
}
redirectTo();
Hope this works for you. Let me know if you have any questions.
Rob