I have used plaster and sand mix before but i used to use thistle board finish, internally there is nothing wrong with it. It could easily be used for filling chases in, but again i would prefer to use Thistle board finish.
The quickest way to fill chases i have found, after giving the old plaster a brush of weak unibond (or similar) is to fill the chase out with Browning, and then skim with Thistle board finish.
With Browning being a high suction coat and thistle being manufactured specifically for boards that have virtually no suction at all, the Thistle dries quickly on the Browning and requires just one trowel up. If you have lots of chases to do all over the house it saves a lot of running around troweling up.
If the build up of Thistle gets a bit too thick, because of the suction. just scrub over it with your wet splash brush and then trowel to a finish. If you use the correct finish for browning, carlight finish, you will need to give at least two trowels and maybe even three, if its a cold winters day.
If your in a rush and need the sand plaster mix to set quickly, just add a bit of cement also, you can get it set in minutes literally. It does take a bit of practice at gauging the mix rate, but its not too hard really.
If you add a good amount of Thistle and make a nice fatty mix, you can even trowel it up as it sets, a home brew one coat plaster mix in effect.