You may need to be logged in to DW to do it, judging by my experiments two seconds ago with a logged-out incognito window, but:

- go to the comm profile
- click "Search for Entries"
- among your options on that page will be a checkbox for "Also search in comments"; check it
- search!

(If you don't have a DW account, you can also just say what you're looking for, and tracking coalies who save their comment notifications may be able to help you find old threads.)