StartupMessage parameters - free-form or not?

Hi,

I wrote a Postgres client and in it I allow the user to specify arbitrary
StartupMessage parameters (Map<string,string>). This is convenient because
the user can for example set search_path without issuing a separate SET
query or encoding things into the "options" parameter. The protocol
documentation also says that the latter is deprecated and what I'm doing
(if I understand it right) is preferred.

A fellow author of a driver for a different language reminds me that libpq
explicitly enumerates the supported parameters in the docs, and I checked
the code, and indeed there is a whitelist and others are rejected. So
technically, he's correct: it's nowhere documented that sending e.g.
search_path in StartupMessage parameters will work, and for that matter
whether everything that you can set using SET you can also send there.

What is the proper behavior for a driver here:
1. Whitelist parameters like libpq does, or
2. Allow the user to send anything, with the understanding it'll work the
same as SET

Thanks!
Jaka