score:3
I don't work for Eurolines, but I am extremely confident that they allow for backpacks in the hold. Unless your luggage is wildly outside of the stipulated dimensions, like, say, you're transporting a two-seater couch, they will accept pretty much anything.
Bus companies the world over tend to be very lenient with what they accept as hold luggage. Regulations such as the one you point out mostly exist to legally cover the issuer in the unlikely event that, say, everyone brings a two-seater couch.
Upvote:5
In 2007, I travelled on Eurolines between Amsterdam and Oslo both ways. We were a group of 15 young people and all had hiking backpacks in the 60–75 litre range. It was no problem whatsoever: backpack goes in the luggage compartment under the bus, small items can be brought to the seat in the main part of the bus.
Although my experience is rather old, it would be shocking if backpacks were banned.