Hello again kapitainsky 
I think settings are irrelevant in this case since I'm asking about a general feature. There are indeed plenty of posts, such as this and this, but none leads to useful result. The former proposes to create a virtual backend to truncate long filenames while the latter proposes a remote database for the long filenames.
Since RClone doesn't support mass rename feature, my current solution is to use a custom Python/shell script to rename the offending files, though it'd be nice to have a native filename truncating support so I don't have to rewrite this on every machine I run RClone on.