I now found a solution: if you book Europcar/Advantage though certain German booking sites (e.g. Cardelmar or Check24) there is a one-way charge of explicitly 0.00 USD. This seems to work between all Advantage locations in the US.
A second option is to book with Hertz via German booking site billiger-mietwagen.de which gives me "no one-way rental fees" for itineraries between California and Chicago (but not reverse) and maybe others, too.
LAX/SFO to/from SEA with zero one-way surcharges (explicit nor hidden ones) can still be had with Alamo via a couple of German booking sites.
Those offers are likely limited to non-US residents, and of course payment is made in Euros (but prices are very competitive anyway, especially if you consider the insurance packages included).