That's exactly what it is, Plan 9 does not have $PATH. You just mount directories with binaries under /bin. When multiple binaries have the same name, the one from the last mounted directory is used, masking the others. You could have something like:

    mount /usr/bin /bin
    mount /usr/local/bin /bin
    mount ~/.local/bin /bin

Since binaries in Plan 9 are usually under additional subdirectories, conflicts are rare. For example instead of 'ping' you have 'ip/ping' which would be expanded to '/bin/ip/ping'. The same way binaries from different system architectures can live in the same filesystem, you just mount the directory for relevant architecture under '/bin'.

See also: https://en.wikipedia.org/wiki/Union_mount

> If this is what it sounds like would it be like making a /path directory and defining it as a union of all the directories containing executables. almost like an efficient and idiomatic way to create a single directory containing symlinks to all of your executables.

That's basically it. Although 'defining' in this case really means 'binding directories into it.'

> How would it handle things like path precedence?

By ordering: first bind the main x86 binary directory into /bin, then bind your custom stuff ahead of it (or after it); as I recall, you could bind to the head or tail.