Are google-internal bugs Buganizer bugs? Sheriff-o-Matic does currently only support bugs in Monorail. I think we could support linking to Buganizer bugs too if necessary/desirable.  

Otherwise, Sheriff-o-Matic does currently already have a comments feature, but if it's hard to find, then we obviously have a UI problem on our end. :)

No, it's a monorail bug, but it's marked as restrict-google.

However, that may have been a red herring, as I had another problem with
the same failure line: it was a master that was down. Unfortunately, it
seems to have come back up because the failure line has disappeared.

Anyway, the behavior when I tried to link to a bug is that it wrote "404
Not Found" when I clicked "save bug", and the dialog stayed up until I
clicked close.

Hmm, I think RVG bugs should be able to work also. Even if SoM can't find Monorail data on the bug, it's supposed to fall back to just parsing for the bug ID from the URL. 

I think the 404 is probably the important hint here. That sounds like maybe there's something off with the key of the alert the bug is being linked to.

So I just successfully linked to a RVG bug for a different alert (clank
compile failure), so the RVG thing is a red herring, sorry...

But the 404 issue was definitely real, and it seems to be alert-related as
you suggest. I'll keep an eye out for it in case it happens again. It
happened on an alert for the chromium.fyi master being down (possibly the
alert text was "stale master data").

Thanks for the info! Annotations (snooze, comment, and link bug) are all based on the same system, so I think this from the same source of another bug, which is that stale master alerts somehow are not having their keys properly encoded in the URL.