The URL value would be hardcoded into the form on the page (which is the coreg form, not the LMP form), so there is no possibility of changing it. But if it can't be passed, then it's a moot point.
Actually, now that I think about it this *could* be done by utilizing the included xtra/signup-xtra.php script with code such as this:
if($list=='1'){
if($custom_refurl) mysql_query("update lm_users set refurl = '".addslashes($custom_refurl)."';");
}
You would then set in your form <input type=hidden name=custom_refurl value=myvalue>
I'm not sure I understand, though, how someone putting in a fake URL could then complain that it is not 100% opt-in? This is still a double-opt in list...even after the info is passed to LMP, they have to confirm before they are added. Maybe I'm missing something.
The 'referring URL' can be faked by someone with enough knowhow and is ultimately unreliable. If someone were to fake the URL and claim you are spamming you wouldn't have any 'proof' about the URL they subscribed from (although, you might be able to determine it if a certain list only accepts subscription from a certain page.. the point is the data is not readily available in the database as expected..)
Faking the RefURL isn't easy for someone with little technical knowhow, but allowing it to be set with a clearly documented var in the signup form would make it easy for anyone with basic knowledge of HTML.
I'd like to leave it out of the regular signup form vars for that reason - personal preference, really.
Let me know if you need help with the custom solution above.
Regards