You want to send a dynamic update to a particular name server, possibly not the one in the MNAME field of the zone's SOA record.
Use the BIND 9 version of nsupdate and specify the name server you want to send the update to as the argument to the server command. Then specify the update with an update command. For example:
$ nsupdate > server ns2.foo.example. > update delete foo.example. TXT > update add foo.example. 3600 TXT "This is a test record" > send
nsupdate continues to send updates to the specified name server until you designate a different name server with another server command or exit.
The BIND 8 version of nsupdate doesn't support the server command, but you can use a BIND 9 nsupdate with a BIND 8 name server. In fact, you can use nsupdate with any name server that supports dynamic update. Dynamic update messages have a standard format, after all.
You can use an IP address as the argument to server, too.
5.21.4 See Also
BIND Name Server Configuration
BIND Name Server Operations
Delegation and Registration
Interoperability and Upgrading
Resolvers and Programming
Logging and Troubleshooting