Since the car's electrical system is regulated to 14.2 to 14.4V, and it takes 14.7V to charge the SLA, a switcher is called for.
Perhaps you could put some big Silicon diodes between the output of the alternator and the car's lead-acid battery (requires a remote sense input to the voltage regulator). This would give you more voltage out of the alternator for charging the SLA, and sufficient voltage headroom for an electronic current regulator.
Another idea: instead of a single 12V SLA, use two 6V ones, and charge them in parallel. While the car is running, you dont need the SLA(s) anyway...