Ok to use electric fence wire to balance HV battery pack in parallel?

Why not try it. I've used 18 gauge copper wire $5 for 25 feet

OOK 25 ft. 35 lb. 18-Gauge Copper Hobby Wire 50161 (homedepot.com)

 

You understand incorrectly, that process is called equalizing, not balancing, and is largely a waste of your time and effort.

To balance the battery or separate modules you need to do a series of overcharges (at a very low current) and discharges (to lower stopping voltages on each subsequent discharge) finishing with a gentle final overcharge.

If you want to balance the battery, there is no getting around using a charger and discharger.

You might be able to get away with doing a 'top charge' which in essence is just doing the first balance charge and then stopping.

 

You should see the tiny Little liars on my Venice hybrid multi-volt charger which goes from around 175 to 400 volts I have a switch on it because the wire is not carrying any amps it's only carrying milliamps so the wires could be Jesus I probably 18 gauge looks like what these are on this charger and it's pretty well built too so I would think you're 12 gauge any kind of copper wire would be suitable on down to around well at least 14 but like I say my charger this thing looks like 18 gauge wire it's very thin is not made to handle any amps milliamps probably I know my charger is 2,400 milliamps at 336 volts

 

By slight, I would expect that the voltages would be 0.1 - 0.2 volts variance.

I wouldn't, but until you charge them all up and do a load test (or let them sit for a week) and see what the variance is, no one can really say. If it is more than 0.3 V, I wouldn't expect the repair to last for very long before throwing a code.

The key to good repair is having 28 modules that are closely matched (both capacity and charge/disscharge characteristics) to each other.