But I do not know in advance what the prefix will be.

You don't need to know it, a prefix is for your xpaths, the namespace is important not the prefix, the prefix is just a shortcut for the namespace

The person writing xml "registers" a local prefix so they can write their XML with less chars

The person writing xpath "registers" a local prefix so they can write their xpath with less chars

Neither person has to use the same prefix to refer to the same namespace