> ut there is a "local" feed on pebble.social that (I think) you can only see if you have an account there.
This is correct, especially for mainline #Mastodon software.
There are forks of Mastodon, like #Qoto (see: https://qoto.org flagship server) where you can follow another instance's “Local timeline” without creating an account there.
The #Fediverse software #Firefish have an “Antenna” feature where you can list the instances you want to follow their “Local timeline” and see them as well.
I think there are other Fediverse software with a similar feature, just not the mainline Mastodon branch (and a few others).