Limit base current on emitter-follower BJT
You may be worrying too much about exceeding a 1mA limit- that sounds more like a limit for accuracy than safety of the chip. Looking at the datasheet it's a recommendation, not an abs. max value. I doubt metal failure modes are going to manifest at a couple mA, and power dissipation is not significant, even 12 channels at 2mA add up to less than 60mW.
A base resistor high enough to reduce the DAC current to 1mA would be in the 3.3K range, so the effect on the collector current in normal operation (gain of 40 minimum) would be less than 10%.
You could also add a reverse-biased diode from each collector to a voltage source (eg. 5V) that would prevent the collector from going lower than 4.3V so the transistor would never saturate (though bad things might happen if the diodes were installed backwards or broke down unless that 5V supply was protected).
But I don't think any of that is necessary.