Looks to me that if you can plant something that the roots are still easily embedded deeper into the soil, then it can hang out the large holes. Maybe some vines.
Succulents usually have shallow roots and so they will easily fall out if root ball is not large enough to be held within the pot walls. When I have stuff planted into strawberry pockets and the plants don't have well developed roots anchor just yet, I use the spray bottle to gently but thoroughly water each pocket. Water may start to run out along with the dirt, but that means you should wait for water to settle into the dirt before continuing to spray water again. Watering in the conventional way thru the top will only cause the water to flow out of the large holes and dislodge any plants not firmly rooted. Large Florist pins (and when they are large, they may be more like landscaping pins) can be used to help hold any plant stem or branch that needs help in anchoring their roots.
Another way to hold soil in is to insert rocks or Styrofoam in netted bags (the ones that onions or potatoes or big ham come in) that can be used to wedge some of the dirt away from the pockets so that you would be dealing only with the plant and its soil when watering. When the plants become established you may be able to switch to watering from the top.
Hope this gives you some ideas...
