You could do this by creating a new ETP 'application' (using the tcl API) that defines a meta-tags field in addition to the basic etp application field spec. (look in the etp source code for guidance - refer to the default application specification code). You would then need to write your own template to return these as part of the >head< of the page when delivered to the browser. The bast way to do this would be to pass the field up to the master template in a variable and have an >if<>/if< in the master template that check for a value and returns default meta-tags if the variable is not set.
[NB. I have reversed the tag separators because tags are not permitted here in a forum post.]