Given that Microsoft's primary target market for RDP is RDS (i.e. being an attractive additional licensing revenue stream), and the secondary is sysadmin work, I don't think you'll find them rushing to support high-resolution stuff any time soon.
There's always Citrix...
(although I don't know if that would solve your particular issue).
Or if you're just accessing a single remote PC, how about Teamviewer ? You shouldn't really be leaving RDP exposed to the internet without additional security (Forefront UAG).