After Googling this for an hour or two and not finding much, I have found something solid just minutes after posting my question!
From page 40 of TRS8BIT volume 09 issue 02 from June 2015 there is an article on a "FreHD Expansion Interface" which includes this paragraph:
If a WDC1771 Disc Controller is found then the LII ROM programs the
chip to ...
This should get you to the Applesoft BASIC prompt, with a checkerboard cursor:
If you don't see that then something is wrong. Otherwise you can check your ROM version:
ROM version 255 can supposedly boot the external drive (though I've never tried this personally) but some disks might not boot correctly. You ...
Simply press RESET (depending on model and setting you might have to hold down CTRL or Open-Apple+CTRL before pressing RESET).
This will bring you into default APPLESOFT BASIC.
Just, BASIC does not contain any disk commands - unless DOS is loaded from floppy - which pressing Reset will abort :)
The only function available is to boot from a controller by ...