I'm not sure if I'm asking this correctly but is it possible (using voyager) to browse "local" of an instance I'm not signed into? Say i'm logged into lemmy.world but I want to browse "local" for, say, sh.itjust.works. Is this possible? TIA
Just go to their home URL and click the Local feed. (In your example, just go to https://sh.itjust.works/) You won't be logged in there, but you can still see the feed. If you want to actually post, you'll need to go to that post on your local instance, though.
Edit: Completely glazed over the 'voyager' part of your question. You might be better served asking on the app community; I'm afraid I don't have an app-specific answer for you.
The Liftoff app can probably do it, I know for sure it can have multiple instances in guest mode (not logged in) & can group the view for all of them as local. Haven't seen other apps that can do that.