I can take ZERO credit for this, but I saw a very useful email today where Jan described how to create a FAST search scope using PowerShell. So I'm just the messenger here but thought it was worth sharing:
New-SPEnterpriseSearchQueryScope -SearchApplication "FAST Query Application" -Name "NewScope" -Description "This is my new scope" -DisplayInAdminUI 1 -ExtendedSearchFilter 'path:starts-with("http://myserver/mysitecoll")'
The ExtendedSearchFilter parameter specifies a FAST Query Language (FQL) expression that will filter the content for that scope.
It can take a while from the scope is created until you can use it in a search center.