Indeed, my solution would have involved something like a TP-Link WR703N, running OpenWRT, powered from a USB battery pack. (I'm assuming that each of the pumps is some distance from the next, and you don't want to have Ethernet cables running across the forecourt. The battery pack is optional, I guess you may also have power available at each pump). Configure each mini router with an appropriate IP address on the Ethernet port, and then do Source NAT from the Wifi to that IP address when talking to the pump. Configure the WiFi interface to connect to an AP that you set up centrally, that your laptop will also be connected to. This takes care of the issue of simultaneous connectivity to each pump.
As far as software goes, it sounds like a virtual machine for each is a requirement, so that they can run independently of each other. This *might* end up being the trickiest part, depending on how much configurability you have in the application that actually does the firmware updates.
* If you can specify the IP address of the destination pump in the update software, great, simply bridge or NAT each VM onto the Wifi network set up in step 1. Configure each VM to speak to the IP address of each 703N, and perform the update. Done.
* If each VM needs to have the corresponding IP to the pump's, things get a little trickier, and you may need to have another layer of NAT to get this to work. I suspect that you would run into issues with the number of private networks that your Virtualisation software supports, in this case.