I don't think a sync plugin would work. Entire parts of CopyQ's architecture would need to be redesigned, which would affect the rest of its ecosystem.

You can read the issue I filed in CopyQ's repository back when I switched to Linux and had to stop using Ditto for the justifications of infinite history: https://github.com/hluk/CopyQ/issues/510

CopyQ is also bound to use Qt's clipboard implementation, which provides the common subset of Qt's supported platforms, with all the consequent limitations. See the other issues I filed for details.