ORDS, SODA & JSON in the Database

I have a OAuth first party cookie-based authentication setup as described here in the first 4 steps of section 3.5.5.3:

https://docs.oracle.com/cd/E56351_01/doc.30/e56293/develop.htm#AELIG90123

which works exactly as described in the example.

But I'd like to create a service which returns "Hello hr_admin"

So replace this:

     p_source => 'select * from emp order by empno desc'

with:

     p_source => 'select <something> msg from dual'

It seems v('APP_USER') is specific to APEX and doesn't work in ORDS, so what is the ORDS equivalent?

Having the untrusted client pass their own username isn't an option, I need it to be validated.