I think (i. e. with no guarantee), that you can use NiMh batteries with say 2.7-3.6V using internal contacts and smart wiring of the middle contact.
But you cannot charge them. Charging technology and response of Ni-Mh and Li-Ion differ.
While for charging Li-Ion you need a constant voltage keeping the maximal current and temperature at defined value (if any of them exceeds, limit the current), for NiMh you need constant current keeping temperature (and internal resistence) in limit (checking for over-charging, better chargers could use pulses and response measure).
However the charging control chip inside Zaurus (Semtech SC801) is Ni-Mh capable (see the datasheet on my pages). In theory, the circuitry can be modified to charge Ni-Mh, but it would need a lot of hardware skills and circuitry study.
The holder on the photo has 4 battery slots, sou you can get 4.8-5V. It is sufficient for the AC jack, and you will not have to shutdown, reboot, set the clock to recharge them (if you keep internal battery as a backup source).