In 2000 I did successfully upgrade a LH-operated (metal)/UA-codeshared (UA flight number) at check-in using a UA/MP-issued 20,000 mile certificate (which was the amount required for a trans-atlantic flight at that time). To the best of my knowledge this was not allowable under the rules. I believe that, in a similar situation today, it is reasonable to ask at check-in if you can upgrade using a SWU. Further, while I agree that such a situation is not allowed under the current SWU terms and conditions (or at least not well defined?), it would be reasonable to expect that the LH personnel would accept the SWU provided there was upgrade space available.