Given that it's embedded as part of the fare rule Combinability restrictions, I don't think an agent can simply override it can book these fares together on one ticket. This topic of open jaws on the EU end (for EU origin fares) seems to come up here once or twice a year. US origin fares to the EU do not have the same restrictions.

It will most likely be cheaper to book a separate positioning flight (so that you begin and end in same country in the EU), rather than to book as two one-ways. One-way PS fares on AMS/LHR-SEA start at around $3400. Roundtrip PS fares on LHR-SEA start at around $1300 and AMS-SEA starts at around $2000.