Inverters change DC to AC converters change AC to DC, but its all in the terminology we use.
I have a portable 12 volt winch in my garage that just hooks over a hitch, I used it on my 4x4 when I had it to pull people out as I didn't like taking the chance of doing damage snatching, for most hings it was about useless even though it was rated for 3k Lb's of pull, the problem with these types of winches is they usually don't have a reverse to unwind the cable at a safe speed, you have a lever on them to disengage the gear box that they free wheel the cable out, this wouldn't be good as it would let the sand bag drop.
From a safety point I would find a small hoist that is more made for the job, most of the kick boxing studios I have been in had a hand or electric winch or bolted to the wall just above the reach of small kids and the cable went up through two pulleys one above it and one above the place they wanted to hang the bag, some of them were powered but had a small AC motor on them and they would power up and down.
The problem with trying to find a 13.6 volt power supply large enough to handle a DC winch is that they will be expensive, the one I have can pull up to 80 amps at 12 volts, you going to pay upward of $300.00 for a power supply that large
So by the time your customer adds up the cost of the power supply, and your cost of doing all the wiring, and the cost of the DC winch (that might not work because of no reverse) he might as well find a AC winch that can be mounted on the wall that is more design for the job and has safety's built in so no one gets hurt, also the cable can be ran through RMC up to a hight that no one can grasp the cable as it is raised or lowered as another safety point or just mount the winch up high with the controls down low, most of the ones I have seen had key switch's to operate them so kids couldn't play with it, I think Graingers has some that might work? I'll take a look.
Another problem with using a winch designed for pulling cars out of mud is they dont have a brake to hold a load, so this could be a problem if the winch lets the load spool down after you raise it.
