Where SOLAS provides that a particular fitting, material, appliance or apparatus is to be fitted or carried in a ship, can the carriage of any alternative be permitted?

Yes. 

Flag State Administrations may approve any fitting, material, appliance or apparatus, or type thereof, or a particular provision, in substitution for one required by a SOLAS regulation if the Administration is satisfied, by trial or otherwise, that the alternative fitting, etc. is as least as effective as that required by SOLAS. (SOLAS reg. I/5)