Send an email to Pollentia properties (and maybe a couple of other estate agents). I think the owner has a few garages dotted around, they offered me a spot big enough for a motorbike or poss a Microcar for €15K plus fees.. The price will be nothing short of insane... I just dump the car at in the far corner at Can Bach car park when I'm here and then leave it at the airport (€800-1200 a year depending who you use). I did ask about buying a parking spot in the port but it was going north of €20K with all the fees. You can find garages in Sa Pobla or Alcudia for €60-100/month but I've never seen any nearby. A local told me they'd heard of €500/month being asked.
Ultimately I think the only way to blag a sneaky spot is to get to know folks of influence in Pollenca...
The other idea I had was to ask the guys at Autos Magraner who I'm guessing own the car graveyard off Cami del Cementeri... maybe you could store it there... if thats what you need.
Is your car nice and shiny or is it a Pollencin car... v important question