Is there such a thing as a USB modem (for backup dial-up use) that doesn't require prior installation of a driver and can simply be plugged into the USB port of any computer that lacks an internal modem and be instantly usable (assuming, of course, that a landline phone connection is available)? I've looked at the Sewell SW-29644 and the Wintec Filemate 3FMUSBMCXV92-R, but I don't think either one of them would be usable in the scenario described above. Advice would be appreciated.
You'll still need a USB driver and a generic modem driver (e.g. pppd in linux). Then you just need a true hardware modem, which is going to be much more expensive than a software modem.
An example of this is the USRobotics USR5637, which is 2 to 3 times more expensive than the options you're currently looking at. Another example are these from radi.com, though these look to be aimed at OEMs.
The reason for the cost difference is that these software modems offload much of the processing to your computer (which is why they require drivers). The hardware modems have chips internal to them that handle all the processing. Note how the radi.com product specifically says "Unlike other software based USB modems which utilize the processing capacity from the host PC’s, the Radicom USB Modems manage modem functionalities themself."