treelectric

As far as I know you are right about no regulations, but there are industry standards covering this, and as you point out, the the overall wiring standards are much less rigorous when you bring 24v out of the control panel to micro switches, etc. The accepted industry standard is now that 'simple' starters can operate with phase voltage to the coil, including 400 volts, but it must not come forward to any push buttons. The push buttons just operate directly on the aux contact of the starting contactor and the overload trip button via plastic rods.
Once you get 2 or more contactors operating with wired buttons it is no longer 'simple', and the accepted industry standard is now 24v. It's usually ac if contactors are involved, because large dc coils have to be suppressed to protect the operating switches against arcing causing premature failure of contacts. While I've been involved in control panel design, (over 30 years), the control voltage standard has gone from 415 volts, to 240 volts, then 110 volts, and now 24 volts ac. 24v dc is still used on banks of small relays, but even here there is still the extra work needed for coil suppression if there is any computer control or plc's in the system, so 24v ac is becoming the norm. Hope this helps.
PS some companies, pepsi cola being one that I know of, actually specify their own unique standards. The sooner we get the equivalent to BS7671 in this area the easier it will be for all.