http://127.0.0.1:8330/?to=domain.com&amount=200.00&comment=order_12345
or
http://127.0.0.1:8330/?to=<bitcoinaddress><separatorchar>1.2.3.4&amount=200.00
But as long as the link is already doing the typing for you, I don't see much benefit in using a domain address instead of bitcoin address. With a bitcoin address, the user can't send an unidentified payment. They can't send payment until they've been given a correct bitcoin address to send to.
What would be nice about sending by domain is you could visually verify who it's going to.
A more crucial issue is what if the browser isn't allowed to connect to 127.0.0.1:
http://bitcointalk.org/index.php?topic=63.msg1589#msg1589
and if that's true, then what about that example freenet link that had 127.0.0.1 in it?