This might not match your overall url schema, but you can set one Meteor app to behind a path like /admin/* by setting the environment variable ROOT_URL to include /admin/.
Then Meteor sets that path as the path-prefix that’s used for the websocket url here:
Alternatively, you can use a local fork of ddp-server with the websocket-url of your choosing. You’ll also need to tell the client where to connect to. This is probably easiest done with the DDP_DEFAULT_CONNECTION_URL environment variable, but you’ll need to experiment a bit.
You can make local forks of core packages by copying the package folder from Meteor’s github into your /packages folder. Meteor always checks the packages/ folder first when deciding which version of a package to use