Setting a psql variable from SQL output

Is there a way to get output from a SQL command or function and have it be set to a variable value in psql? My main goal is to execute a certain script depending on the version of the database server (either 8.4 or 9.0). I can dynamically execute a script using the following, but I can't get it to set the myscript variable value based on the version of the database:

\set myscript 'psql_script.sql'
\i :myscript

However, I would like to be able to have the script executed like this:

\set myscript = 'select substring(version() from 12 for 5);'
\i :myscript

I can obviously select the version into a PL/PGSQL variable, but then I can't execute the script from PL/PGSQL.

Thanks,
Bobby