On the filters tab, there's a "Show only traffic from..." with a drop down showing all current processes. Linqpad has an MDI interface that changes the process number each time a script runs so it's impossible to filter on a particular script. I suggest being able to filter by substring of the process name field.
Right click the process name in Fiddler's main session list and choose the Filter submenu. Or you can use FiddlerScript to easily add a filter.
Oops. The context menu filters have a "Hide: foo:*" but not "Show only: foo:*". Maybe the latter should exist.