Using a tablet without battery (and making it believe it has one)
Your solution may be as simple as modifying the charger output (installing a resistor to cause a sufficient voltage drop so that the charger output will be equal to the Li-Ion battery output and then connecting the modified charger output to the B+ &amp; B- solder points or wiring on the tablet circuit board.

If you have a bench DC power supply, before you hack the charger, you could touch the leads of the bench power supply (properly pre-adjusted of course) to the B+ &amp; B- pins to determine if that works.
B+ and B- are on the board, hidden at the top of some removable phone batteries. You unpeel the sticker from the top where the wires or pins come from the battery, and then pull the black plastic sleeve. There you will find the tablet's/phone's B+ and B- connections. Just remove the battery and solder to those two pins

Good luck!