There does not seem to be any combination of escaping which works with v0.2.2 for using config:set with a value with spaces, ex:

sudo dokku config:set myapp JAVA_OPTS="-Xmx1024m -Xms256m"
sudo dokku config:set myapp "JAVA_OPTS=-Xmx1024m -Xms256m"
sudo dokku config:set myapp 'JAVA_OPTS="-Xmx1024m -Xms256m"'

etc.

There's a bunch of logic in plugins/config/commands which uses whitespace or space characters as break points