i think it will work, atleast i've done this accidentially at a customer (new appserver w/ the old keytab while the old appserver was running). the instance name doesn't seem to matter - it doesn't seem to have to match the hostname.
Using the same bladelogic.keytab file for multiple appservers is allowed, and as you indicated, is necessary if you clients are operating behing a load balancer and are directed to any one of multiple appservers. The keytab file contains the decryption key the appserver requires to decode and authenticate the Kerberos service ticket presented to it by the client. The service ticket is generated by the domain controller, which has no control over which of multiple appservers the client will get directed to.
The instance name can be anything (it is just convention to have it be the domain name of the targeted server). Feel free to specify the virtual IP. Note that is just gets interpreted as a string that is used to index entries in the keytab file.