Also, what would the HIGH-level output voltage be at this current? - the Data Sheet just says "TBD!" :-(

Hallo Andy,

you can estimate from data given at IOH = -3.2mA. For this current VOH = Vdd - 0.4V, typically. So, source drain on-resistance is something about 0.4V / 3.2mA = 125 Ohm. Assuming that at IOH = -20mA no saturation occurs, output voltage should be about Vdd - 20mA x 125 Ohm = Vdd - 2.5V! If saturation occurs, which seems to be very probable, output voltage is even smaller.
Because Vdd can be in the range between 2.4V and 3.6V, this data is also highly dependent on Vdd. Means, while at Vdd = 2.4V output voltage will totally collapse, at Vdd=3.6V output voltage could be in the range of about 1V, or so.

Keep in mind, that pulling-down output transistors provide much lower drain source on-resistance, namely about 30 Ohm, which is more than four times smaller. So, output is able to sink 20mA with adequate voltage drop, but not to source!

Hope it helps,
Kai