Renting a van for transport of people is in general not that common in Spain, so you may find it difficult. If you are talking about renting a van for transporting "stuff" I know I rented one from Europcar once... seats two in the front and you have a big room in the back to put stuff (I used it to move).
I've not travelled from Ferrol but you will only be allowed to take bike on Regional trains or Regionales and from my experience last year there is often only 'official' space for three bikes, though the commuter trains into Santiago had space for 5 - 6 as I remember.
There is a narrow gauge train on the Ferrol route and I reckon space will even more at a premium on this.
There are two through trains a day from Ferrol to Oviedo 8am & 3pm
taking 6 hours 30min and from Oviedo to Santander at 9am and 3.30pm
taking about 5 hours. It would be possible to do the journey in one
day but if you have the time Oviedo is a town well worth looking
around and part of the history of the camino.
The total fare is about 28 euro.
"FEVE Policy for Transport of Bicycles
FEVE as general norm admits the transport of bicycles in all the
trains with a maximum number of a one per traveller and a different
maximum per train according to CCAA and different lines that vary
between a minimum of three by unit train and a maximum of twelve.
They will be located, depending the train, either in the van or the
carriages and they must be clean so that they do not affect the
driver or the rest of travellers. The loading as the unloading will
be in charge of the traveller. The transport of bicycle is considered
as hand luggage. In order to avoid problems in use it is best to for
the traveller to consult with the information services."