iRule to Redirect to a Specified Pool

We have a lot of customers, and it continues to grow, who connect to our servers in the data center using Microsoft RDP and an assigned URL. The customers connect to the URL we provide them using the RDP client on their local desktop. The public IP assigned to this URL is NATted to the virtual server IP address on the LTM. From there, they are directed to the assigned pool. Each customer has their own unique URL which has an unique public IP address. So, the problem is we are using up a lot of public IP addresses. What we would like to do is assign one public IP address to multiple customer URL's and then use an iRule to redirect these connections to the correct pool based on some value obtained from the RDP TCP payload. I know RDP sends a routing token that contains the username, but that would not work since there could be a "jsmith" at any or all of the customer sites connecting. So, is there any other value that can be parsed from the RDP TCP payload which is unique to the customer that can be used to determine which pool they get redirected to on the LTM?