According to a little research, the Japanese Snowbell will self-seed lightly, particularly in damp soil.

So I wouldn't recommend inclusion in leaf compost, but a compost pile that heats warm enough to sterilize the seeds should be OK if you turn it to assure that all seeds are killed.

Of course, if weeding them out doesn't bother you, go right ahead and do so. Some will rot and add their mass to the compost, some will sprout and can be added to your regular compost pile after removal.

Thank you so much for doing the research. I couldn't find info anywhere. I'll put them in my hot compost.