Yes. A UA SWU can be used on a stand by basis on any LH flight. I've used one from FRA to LOS (Nigeria). And others have posted success in using them from FRA to India or other points in Asia, which is a good use of an SWU in my opinion.

Be careful of wasting an SWU (or miles for that matter) by upgrading short intra-European flights on LH. There is little difference at all between economy and business, particularly on narrow-body aircraft.