French-only Montessori preschools?

No and there are no local French schools which are officially montessori either. All Montessori schools are private and supposed to be bilingual, in reality a child may spend several years there and not become bilingual in French, there are few French speakers (other than the teacher) at many of these schools so simply not the complete French environment.
Here is a map and list of all international schools in Brussels / Waterloo are and all the "bilingual" Montessori schools are on there - starting price about 10k for full time.
http://batchgeo.com/map/0777fe5f939fe09f11d58a91ceee1066