Has anyone got recommendations for a battery powered metronome/speaker that could be used to emit a regular sound over a period of ~1 week and a radius of 100-200m outside, so that it could be used to time-sync 4 or 5 recording units for localisation?
have you considered using a Minirig 3? These are super powerful yet compact (W = 101.6mm H = 73mm), high quality speakers with built in amplifier (40W), big on the festival scene. Battery life is around 100+ hours. You can connect to it via bluetooth or a 3.5mm lead.
I've used one in the field before and it was great, with some minor weatherproofing. For producing a time-sync signal I think this would work.