I was able to get f29/f28 builds going for initial/experimental gtk3-kde5 support built in libreoffice-6.1.x, https:://bugzilla.redhat.com/1647233
here's the fruits of that labor,
https://copr.fedorainfracloud.org/coprs/rdieter/libreoffice-kf5/
My initial testing was good, just verified that kf5-based filepicker shows when saving a file if libreoffice-kf5 package is installed and run from within a plasma session.
-- Rex