Force legacy file chooser definitely should work - are you using the latest version of Pleco? What exactly happens when you use it - do you still get that note about enabling the Documents app, or is the problem just that you can't access cloud storage services?
It's certainly *possible* to access cloud storage services outside of Documents, but it's a ton of work to support; not unreasonable for a dedicated file manager app like ES File Explorer, but for us, making an app that mostly does stuff other than work with files, it would be hard to justify going in and adding support for each cloud service's API just for that one small aspect of Pleco and just for the small % of our users on Xiaomi phones.