Keep in mind the practical aspects of the installation.
The fire alarm system isn't going to be removed after the warranty period; the fire alarm system is going to be in place as long as the building stands. Not only that, but the Booster Power Supply (BPS) requires regular maintenance, including replacement of the batteries every three to four years.
Junction boxes, with wire junctions that don't have to be looked at for many years, aren't a service problem. Boxes that have to be accessed on an annual basis and have to be serviced regularly become a service problem if they're hidden above the ceiling and have to be accessed by ladders or lifts.
The extra service costs are not paid by the contractor, the extra service costs are not paid by the installation company, the extra service costs are not really paid by the service company, the extra service costs that start occurring after the warranty period are passed on to the owner of the building. It's the owners of the building that will be paying for the extra cost of maintenance for as long as the building stands.
Technically, to be serviced properly, the BPS is an electrical panel that has to be located in an accessible location. If you check your NEC (NFPA70) code book, there are references to electrical panels and the required accessibility of the panels.
I'm not sure of the exact location of the rules, but don't BPSs fall under the electrical control cabinet rules of the NEC (NFPA70)? It does have a 110 volt connection to it.
Check with the Electrical Inspector (this may or may not open a can of worms) and ask the inspector about the mounting of electrical control cabinets, with batteries and electronic fire alarm control equipment, and see what the inspector thinks. The NFPA is really only guidelines to follow; the electrical inspector is the authority who makes final decisions.