No one has replied... I'll try and help
I assume you have Tableau Sever?
You can certainly add filters to a tabcmd, to return a specific part of the data, but it does depend how the worksheet (which feeds the dashboard) is set up.
I can imagine, you may have to create a separate copy of the report, purely for export, but it may also make your life easier!
You can find a little about passing filters via tabcmd here
But before all that, i want to check if
a) its still outstanding
b) you have tableau server
c) assuming yes so far, that you can login etc via tabcmd?
Once you're at that point, then it becomes about the filters/ parameters.
if you're not at that point, we need to get you there first!
Let me know