I am certain for sure that bag transfer is allowed even if you register in any albergue. I researched this quite heavily before I decided to accompany my daughter through the camino since I did not intend to walk the entire trip. The rule to stay in an albergue is if you walked the camino rather than ride a van or other forms of transportation like bus, train or taxi. I just 'accompanied' my daughter from Saint Jean to Roncesvalles, and a piligrm couple asked me to carry their backpacks while they stayed in the Roncesvalles albergue (a sophisticated one I must admit which you should definitely try!). I did not walk but took a taxi since I was sick. For any bag transfer from town to town, simply go to a major hotel in that town (the night before) and enquire how to transfer your bags to your next destination town; they usually tie up with bag transfer entities as it is an ongoing business. Also, if you are sure beforehand, the website "follow the camino" has links where you can pay ahead if you are sure which towns you will be stopping to sleep; however, this may not work for many since your sleeping towns may change once you determine your walking 'power'. The rate for each bag is 10 euros and cannot exceed 20 kg (or 44 lbs - international luggage limit). Hope the above helps.
Buen Camino!