I have seen that done quite often. If there is some extra wire in the ceiling but not enough chain, go to a lighting store or hardware store and buy a length of chain. Just use pliers to bend one link on the existing chain and one length on the new chain and splice it in, then squeeze back together with the pliers. Don't worry about scratching the finish on the chain since it will be several feet over your head. If you don't have enough wire, it's a simple matter to change it out. Shut off your breaker then remove the fixture by loosening the nut just below the round ceiling plate. Then the round piece with the loop at the end of the chain will come off the bracket in the ceiling. Undo the tape and wire nuts of the original wiring and take the fixture down. Now you can buy new cord and reconnect the wire from the inside of the fixture to the ceiling. If you have a Habitat for Humanity Restore near you, it might be cheaper to buy a fixture with the right length of wire and chain than to redo the one you already have. We once had a customer buy a light fixture, remove the brass chain and immediately redonate the fixture itself

Since your customer wants to muck about awith non-standard, it's up to them to tell you what they DO want. Don't fall into the trap where you make the suggestion that they can later disagree with after the fact and want changed.

I would say 12” to allow for winter coats.

You can also attach a piece of hardwood into the studs at the outer edge of the wall and hang your hooks onto that. We did that for our entryway knowing that all the kids' heavy backpacks would be hung there. We painted it in contrasting color.