If you’re generating the GCode from LightBurn, you may be able to push the axis length change as a start Macro, or by editing your offline file.
One of the Devs feels this may work for the extended bed.
The $ commands are usually suspended when the engraver is operating, but if the macro arrives before the engraver thinks it’s operating it might work. You’d have to test.
If that doesn’t work, please grab a working offline file, make a copy of it, change the name to “Test” then Empty it and add your $131=700 command and put that in the offline controller and “attempt to engrave it”.
If they haven’t locked it out, that might work.
Please let me know what you discover.