You could only do this with a collection project if they worked with observation fields; then you could add an observation field (e.g. one of these) indicating that observations have cotyledons and they’d automatically be filtered into the project. There’s a feature request for that here. In the meantime, you’d need to use a traditional project. Or just use the already-existing observation fields if they serve your purpose without needing to create a project. Adding to a traditional project takes just as much effort as adding an observation field anyways, so they seem equivalent to me.
Fasciation and 4-leaf clover projects would qualify for the latter description I think.