"I have a couple geeky questions:
"If I have a 72V battery pack on my bike and want to step down the voltage for some lights, what bit of hardware do I need to slip in?
"And if I step down the voltage to 24V, would that allow me to power 2 12V systems (front and rear lights) in the same circuit? Or do I need to use 2 separate transformers or whatever to get down to 12V for each system?"

I would assume a 72v battery pack was made up of a bunch of smaller units generating 1.2 or 1.5 or 2 volts each. The ideal way to drop that to 24v is to rewire it so that you have three sets of units each producing 24v, or 6 sets producing 12 volts, and and then wire the sets in parallel with each other. If this is a single factory-made battery that produces 72v, I'd probably just look for a different battery, myself. If this is a rechargeable battery, you may have trouble finding a recharger. If it's a drycell type thing (seems like I used to see those in stores long long ago- big long battery about 8" long), then it'll be expensive.

Transformers work on alternating current, like house current. You can't just use them on a battery unless you have some way to convert the DC to AC, which gets awkward. I think there are transister units that can do this kind of thing with low voltages (like 12v to 5v), but I doubt you'd find much to help with 72v to 24v.

If you get down to 24v, you can power 12v front and rear units IF they have the same wattage/amps. But normally, a headlight would be much more powerful. If you connected the two in series, the headlight would burn very dim, and the taillight very bright until it burned the bulb out. Essentially, the high resistance in the taillight would limit the current through both of them, and would also cause a bigger voltage drop across that item. And if either bulb burns out, both go out.