Visited yesterday and did a short charge
The chargers are in the delivery area of the AKB, before the ticket gate for the customer and employee parkings.
There are signs saying those parking spots are only for use during charging, but no mention if reserved for customers, employees or everyone. The Swisscharge QR codes are installed.
You enter through the same ramp that leads to the short term train station parking, labelled "Bahnhof + Kantonalbank". Mid-way down the ramp there's a fork where you can turn left towards "Kantonalbank". Then there's a large delivery area with many reserved parking spots and the ticket gates towards their paid parkings. The 4 green charging spots are between the 2 different ticket gates, outside of the paid parking.
If you enter and all the charging spots are occupied, you can turn around and exit without having to go through the paid paring.
It's not super clear to me how you are supposed to exit on foot. There are a few signs, but no foot path painted on the floor. You're not allowed to walk up or down the entrance ramp. The signs seem to indicate you need to walk on foot through the ticket gates inside the paid parking area, then use the foot exit for that parking. Since you are not in the paid parking you don't have a ticket so I'm not sure if you can re-enter the parking at any hour. The paid parking closes between 02h and 05h in the morning so I assume the staircase cannot be used at those times, but you could still access the chargers at those hours by car since there's no gate.
I'm including a picture of the parking tariffs for the paid parking just for reference.