BTW if you want to rent a road bike in CM, unless you're a fan of classic bikes that might break down, you really need to rent an er6-n/Ninja 650/Versys. All the other big bikes for rent are anywhere between ancient and old, and in an unknown state of repair. Not worth it.
I doubt anyone would hold a bike for you - that's not how it works here. Luckily there's many places to rent big bikes, even more so since new Kawasakis have become available in Thailand. "Not necessary" is a Thai euphemism for "we don't do it".
Not to say Mr. Mechanic was in the right in this case, just correcting some assumptions people might have before renting. If you know that nobody's going to rent you a new or even semi-new CB750, and that nobody's going to be able to hold a bike for you, you might save yourself some disappointment.
I'd never rent a CB400 here - while you might get lucky, all the ones I have seen were total crap. The rental shops obviously won't tell you that....
[1] Holding a bike seems trivial but in order to hold a bike you'd need to write that down somewhere, staff would have to be aware of what was written down, and would need to make sure people return the bike before that date - no shop in CM is going to bother doing this, and even if they try staff might not be aware, the customer might keep the bike longer, etc.