i don't know if it would help all plants, or all plant sitters, but I found that a capillary mat helped me to over-water my plants less. If the soilless mix is very sandy and gritty it might not wick well enough for bottom watering. You would have to test it out before going on vacation.
I use cotton flannel, but a piece of toweling or Tee shirt should work as well.
The mix has to extend all the way through the hole in the bottom of the pot to make contact with the flannel (there has to be a continuous capillary path from the soil in th4e pot to the flannel in the saucer).
Tell them NOT to pour water on top of the soil.
Only add water to the flannel.
only add water if the flannel is dry.
Don't add more than this much on any one day (give them a shot glass or small liqueur glass).
One advantage is that, if the pot IS too wet, the flannel will help pull perched water down OUT of the pot. It's as if the capillary attraction between water and flannel is added to the force of gravity in pulling water AWAY from the soilless mix.
