Hi Jack !
Yeah effects routing is something i had not yet implemented at the time
of release of the editor available at
http://miprosoft.com/mpxg2
My G2 is in standby and the project also as it did not bring me much (ie
$120 in 6 years for +2 months of hard work instead of the salary
expected - but yet really appreciated !)
I have other priorities today (working to sustain my family) and I
prefer to plug my guitar directly in my Carvin XV212 (sigh needs
repair...) or grabing the cheap Martin I love (so I'm not in a hurry).
I believe you'll have to use the knob.
The manual is available on
https://lexiconpro.com/en/products/mpx-g2
See section 3-10 about Routing Map
Alternatively the source code for the editor is licensed under GPL and
available on github:
https://github.com/luxigo/Lexicon-MPX-G2-Editor
If you can find somebody (you?) willing and able to add this functionality for free I can help.
Also you can hire me I would do it myself with pleasure.