in a collection project, you should be able to accomplish this by using both a taxon filter for a particular taxon and taxon exclusion filters for all the immediate children taxa.
there is a feature request to add this functionality, but the feature currently doesn’t exist.
you could add these to a traditional project and then combine the traditional project with the collection project under an umbrella project. this would effectively give you your OR condition.
i’m not sure how many observations you would want to add to such a traditional project. i assume since you’d have to add the observation fields manually probably, then you could also add them to the project at the same time. but if for some reason there are too many to add manually, it would also be possible to add observations to a project (and add observation fields, too) via the API, given a list of observation ids.