Weapon customisations may only be applied to a piece of equipment that is part of the Standard Regimental Kit or a favoured weapon of the soldier's Regiment. - OWCRB, page 190.
Since Armour, Artillery, and Mechaised Infantry (among others) start with vehicles in their Regimental Kit, can they apply weapon customisations to the weapons of the vehicles?
As examples, consider:
Apply Custom Grip to the pintile-mounted Heavy Stubber on a Leman Russ or Chimera.
Apply Fluid Action to the Autocannon of a Chimera.
Apply Extended Barrel to the Earthshaker Cannon of a Basilisk or the Battle Cannon of a Leman Russ or the Autocannon of a Chimera.