I had a 232-to-USB converter, so here's what I know so far.
The 8226 logs under XP using an Aten serial bridge USB converter. No idea how common that may be.
The MS8226 logging software that came with the units is "DMM v2.02". I previously looked at DMM v1.00 or some such. Seems the same.
The software lets you select one of four COM ports. Windows may assign the USB adapter a port outside the software's range, in which case you need to fix it in the Device Manager.
The MS8226 does work with the direct-to-USB cable that shipped with my MS8128. It does not work with the 8128 software, though. I don't know if you can even get the cable as a separate item, although it would be nice.
You can get both the 8218 and 8226 working together on the same PC, at least under XP. Getting both started is tricky, as the 8128 software seems to crash any time it sees incoming information that it does not like. The 8218 does not use a fixed port assignment - instead, it attempts to auto-detect which port is being used. This means it can accidentally find the 8226 meter and crash.
(It also seems the crash-on-start for the MS8128 software is due to it not recognizing the information coming from the 8128 DMM. It seems it's best to start the software, press "Start", and then start the RS232C output from the DMM itself. This maximizes the chances of successful synchronization.)
Usually, you have to get the 8218 started first, so the software will detect the right meter on the right COM port. You can then fire up the 8226 software and use its fixed port assignment to detect the 8226 meter.
Given this I don't see any reason why you couldn't run one 8218 and three 8226s on the same computer. I only have one RS-232 to USB converter right now, so I can't try that yet.
They will work through a generic usb hub.
The 8226 DMM v.2.02 has some good features lacking in the 8218 software, such as the ability to assign the COM port and change the logging interval from ~2 seconds to several thousand seconds. It also has better prompts and error/status messages. Unfortunately, I can't tell how big the buffer is.
The 8226 also has more attention to detail on the logging function. For example, the 8226 will auto-cancel the sleep mode when the RS232C output is active. The 8218 does not, although you can easily cancel it manually.