Parking a bike with wide tyres in a narrow bike rack
The solution in my particular case turns out to be that the 2" Marathons are actually no wider than the 1.9" knobblies - when fitted on my wheels and at my current inflation pressure of about 3 bar, at least - so the bike still fits in the rack. 

This won't help anyone whose tyres actually are too wide to fit in a narrow bike rack, but it may help someone in a similar position to know that tyres may be slightly narrower than their nominal width.

In a year of riding on the Marathons, incidentally, I've had one puncture, compared to about one a month on the standard tyres. I also found the difference in rolling resistance compared to knobbly tyres was surprisingly noticeable.