You should be able to change the threshold to only include the nodes you want.
Under properties of the "CmaTftpHeartBeatProblem" threshold conditions, there should be a tab called "Nodes to Montior", currently it is using the node group "#All Telerphony Systems/Cisco CallManager Appliance/All Nodes", you can remove that and add it all the nodes that you know are actually the TFTP servers.
Does anyone know the answer to this question? I am looking to be able to access these META fields in either a Display and/or a threshold. The question is "Can we access META FIELDS from within a where clause in either a Display and or a Threshold."