Attached is a draft for the use of Ted Krovetz' UMAC (RFC4418) as a SSH MAC. OpenSSH -current implements the umac-64 method described in the draft under the name "umac-64%openssh.com@localhost".
Have you also looked at Ted's new VMAC algorithm? It's still in Internet Draft stage, but it's simpler than UMAC and is also significantly faster on many platforms, especially 64-bit platforms. Public domain implementations are available from http://www.fastcrypto.org/vmac/ and http://www.cryptopp.com. (Disclosure: I contributed several ideas to the current VMAC design and am a co-author of the VMAC draft.)
Both UMAC and VMAC require unique nonces. Using the sequence number as the nonce as in your draft may cause nonces to be reused if someone takes a snapshot of an active SSH connection running an a virtual machine, and when that snapshot is restored, the SSH program sends out new packets before realizing that the connection is no longer valid.
Unless there is a good reason to believe this can't occur, it would be safer to use random nonces instead.