This is such a fun question! I love that succulents can be planted in just about anything, and bowling balls are no exception!
I would use succulents that have a stem such as Echeveria, some Sedums, Graptoveria, Portulacaria, etc. You can take a cutting of one of these plants, fill the hole of the bowling ball with coconut coir, and then place the cutting in the hole.
You’ll want the cutting to go at least half way into the hole to prevent it from toppling out.
All of the succulents mentioned above are not cold hardy and would need to be taken indoors during the winter. At least for any period where there is a possibility of freezing.
If you want to stick with cold hardy options, I’d recommend using Stonecrop Sedums such as Sedum reflexum, Sedum spurium, etc. These will be just fine outdoors over the winter.
Note to readers in the US: You can purchase a cold hardy succulents from Mountain Crest Gardens by clicking here.
You could also try using Sempervivums, but I’m not sure they would stay in place very well in a bowling ball hole.
As far as watering goes… I’d pour in a tablespoon or so of water in the hole every week or two. It will be hard to check if the soil is dry.
Once the succulent is really rooted though, you could even pull the whole thing out of the hole, soak the soil, and then put it back in. Just pull it out to see if the soil is dry before watering again. That would be kind of fun!
Sounds like you have a great project on your hands!