hi TK
It's true that USB dongles tend to be bad at roaming, but you may be able to make it work with some tweaks. First some questions...
1. what is the dongle brand/model ?
2. what is the OS of the device that the dongle is connected to ?
3. if windows, have you tried checking the device settings, eg. network connections -> right click on the USB device -> properties -> configure and have a look, there might be a roaming agressiveness or similar setting.
4. does the dongle share a network with other devices, or could it be made to use it's own SSID if you had to ?
5. would anyone care if the device was forcefully kicked out when it has low signal in order to let it reconnect (hopefully to a stronger near by AP).