[wilywyrm]

I experimented some more with the touchpad, and yes, it discriminates based on the presence of the voltage divider. (Wall of text, sorry.)

The setup was just my computer, in three different configurations. Everything besides what was on the data lines was kept exactly the same to see if it affects anything.
The first was simply plugging in the cable. On a power saving configuration, the touchpad sucked in a net total of 360mA after turning off the screen. Didn't check voltage under load this time, but I remember it being 4.6-4.8V last night.
For the second, to simulate a dumb charger that does nothing to data lines, I split a cable I had lying around and cut the green and white data lines. This time around, it only took in enough current to prevent battery drain, so the current hovered around 0 mA, sometimes going up a little, sometimes down a little. Voltage was 4.96V. The touchpad probably does this to avoid damage to itself and the charger from drawing too much current.
Lastly, I mimicked the AC charger and measured the current with the voltage divider on the data lines, and it was a little over 650mA, with a voltage of 4.46V.

Normally with the AC charger, the touchpad will draw in somewhere from 1400mA to 1800mA with the screen turned off, so it appears to me that the touchpad's thought process is to be cautious with computers and non-HP chargers, and to suck in as much current (up to ~1800mA) without causing the voltage to drop too far.

Whether or not any given charger will work depends, but odds are little to none that any given generic charger will have this specific necessary circuit for the touchpad. You can try to charge with them, but the touchpad will only draw about enough to match what it would take from the battery otherwise, or at the very most, not anywhere near the official charger.

Also, for those who are looking for compatible chargers, please note that some are simply transformers in a plastic shell. These cheap chargers will have voltages lower than what it says on the label at full load (power draw), so they may not even supply a high enough voltage for the touchpad to draw in at that full advertised load. You can still most likely slow down the battery drain while in use, however.
A charger rated at anywhere from 4.75V to 5.3V should work. The amperage(A) rating should not matter, and the more, the better. The extra current will not force its way through, and as long as the charger pumps out power, it should at least slow the draining compared to leaving it on stand-by.

For everyone, if you are charging while using, it might speed things up a bit if you changed the CPU governor from "ondemand" to "conservative" or even underclocked the CPU from within govnah so that the CPU doesn't ratch up straight to full and suck up power when you turn the screen back on to check the charge process.

That's all well and good, but if that's not good enough for you, you can always drive on down to radioshack and pick up some supplies to go modify your own.

All this, however, may not matter if you decide to put Android on your touchpad. The check for official chargers could either be something programmed into the tablet to check the data pins when power is applied through the port or something wired into the hardware (e.g., a chip that checks for this under the same circumstances.). The former is more likely, but may not be patchable, and the latter would require the same setup under Android.