building extension with large string inserts

I am trying to build an extension where there is the need to insert large
strings consisting of javascript code. The easiest way to get these string
currently into a table is by using

\set varname `cat mycode.js`
INSERT INTO mytable VALUES (:'varname');

and run this from the psql client.
psql will escape the string nicely and stuff it into a text field.

This does not work with extensions since I cannot use \set anywhere else
than the psql client.

What would be a proper way to get this code into a table via an extension?
I've been working on generating INSERT statements for the extension's sql
file but it seems a tedious job to escape the code myself.

Best,
Tom