Can I solder 5V USB to a tablet battery connector?
In the past while debugging power management on portable SOCs, I have removed the lithium ion battery, connected a power supply with logging capability, and then connected the battery thermistor so that the PMIC still thinks the battery is present.  You do need to provide the correct voltage, or else the PMIC may think the battery is catastrophically over voltage and turn off.  Without knowledge of your specific device, I cannot tell you definitively if this will work for you, but it is plausible.

It may still be better to use the USB connector however.  You can ignore the data pins, so it is often very simple to desolder the existing broken connector, and then find somewhere under it where the +5 and GND traces are accessible even if the traces have lifted off the board.