After much trial and error and some more research, here is how I got it working, in case its useful to somebody else:
Create a free account at OpenSubtitles.COM (do *NOT* use .org, it must be .com). The .org version looks similar, but the app won't work there unless you pay for premium.
Install the OpenSubtitles.COM app from Kodi repository (once again, do not install the .org version). Configure the app with username (not email address) and password of the account you created.
I was still getting the vfs.libarchive error, and because i am not running the latest version of LE I cannot access the LIbreElec repo. So I pulled a sneaky and went to http://addons.libreelec.tv/11.0.0/ARMv7/arm/vfs.libarchive/ with a browser and copied the vfs.libarchive-20.3.0.3.zip to my device over the network, and then did a install add-on-from-zip-file. That was the right file for a OrangePi3LTS, yours will be different.
I tried a handful of shows and movies, and so far the only one it couldn't get good subs for was a really really old movie. So I am pretty happy with this.