I have seen partially deconstructed bikes being placed in the luggage hold of buses/coaches in Spain but they will obviously give preference to bags and suitcases.
Have you thought about following the Japanese "
rinko" method of bike packing? Think of it as bicycle origami!
In my local city bus station there's a cycle shop and, while in there buying a new inner tube last summer, I got talking to a couple of Japanese students who were taking their bikes to pieces in order to fit them into Rinko Bags (except one of them had lost hers and was using an IKEA blue trolley bag instead) to take on a cross-Channel coach.
The bikes are set up so that you can easily remove both wheels, the pedals, even the front forks (!) and it all goes into a tiny space
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The ladies said they had it down to less than 20 minutes to disassemble/reassemble but if guys tried to "help" (ie take over) it usually took much longer!