I think you guys would be doing yourselves a great favor in getting your cars serviced while in Europe rather than trying to keep the mileage low or squeezing the trip in without the service by some other means.
Having done ED with a 335i, I can tell you it was much more fun once the car was broken in. With an M3 it would be even more of a tease not to be able to take the car to max RPM and top speed. I would plan ahead (far ahead) and schedule your 1200 mi service with a trustworthy dealership along your route. There's no reason why you could not have the car serviced while you were out sightseeing or doing other activities for the day. I honestly can't imagine doing ED and never being able to take full advantage of the car.
2008 Interlagos Blue E93 M3 - M-DCT
2011 Salsa Red Jetta TDI Sportwagen - DSG