Andrew Hatfield has described very well what needs to be done when running multiple SQL Server instances on a cluster.
.. you are already using dns aliases to support your application (which is a good thing)
Excellent - this is the way to go.
... why do you want to try and make life a whole heap more complicated by trying to do what you're describing here? It is much simpler, and supported, to simply set each instance to a unique port. That they are in separate cluster groups gives them separate IP addresses.
Agree with Andrew.
From memory, I don't think the SQL Network Configuration Utility provides an IP address option for your virtual (cluster group) IPs.
Andrew is correct that IP cannot be specified but the port can be specified.
You need to take only a few more steps to have the SQL Server instances have fixed ports rather than dynamically assigned ports and to map the dns aliases to the actual SQL Server instance names. This will insure that the clients can connect using only the DNS Alias and do not need to specify IP or port.
First, decided what ports are going to be used for SQL Server. Registered Ports are those from 1024 through 49151 and Dynamic and/or Private Ports are those from 49152 through 65535. These ports number ranges should not be used for named SQL Server instances as a future conflict is possible. Reference http://www.iana.org/assignments/port-numbers
Second, so that no other program requesting a dynamic port will be assigned your desired SQL server port, reserve the desired port numbers. "How to reserve a range of ephemeral ports on a computer that is running Windows Server 2003 or Windows 2000 Server" http://support.microsoft.com/kb/812873/
Third, "Configuring SQL Server Named instances to use a fixed port"
http://technet.microsoft.com/en-us/library/ms345327(SQL.90).aspx. This will require a SQL Server restart.
Fourth, map the DNS aliases to the SQL Server using the SQL Server Configuration Manager.
Finally test by connecting using the DNS Alias without a port number from a different server.
Good Luck
SQL = Scarcely Qualifies as a Language