If you want a solution not involving outside consulting help, I suggest just making three buttons and controlling them with form rules and / or in the formula itself.
ie
IF([Department]= "XYZ",
"javascript:void(copyMasterDetailButtonHandler('&relfids=72&recurse=false&sourceRID=3&destrid=" & [Record ID#] & "', 'bmzg6sw35'))"
)
Alternatively, I do have a native solution not involving javascript, but it would probably take an hour of consulting time one on one to explain it.
Basically you would push a formula URL button which would populate a two fields called [Focus Department] and [Focus project] in a single record with Record ID # 1 in in a table called User Focus. That would then be looked up to a table of tasks. Then that same URL button would next execute API_RunImport to copy the records from a table of tasks which match the Department into the child tasks table for the focus project.
Then the URL formula button would redisplay the project.
The button would look like this
$URLONE
& "&rdr=" & URLEncode($URLTWO)
& URLEncode("&rdr=" & URLEncode($URLTHREE))
