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.
Oops. The context menu filters have a "Hide: foo:*" but not "Show only: foo:*". Maybe the latter should exist.
For now, the simple approach is to click Rules > Customize Rules and add:
Or add a menu as seen in https://stackoverflow.com/a/28080377/126229
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.