I think journal posts are a great idea. One advantage is that the people identifying observations in the project can then link to the post to explain why only a broad ID is possible. This can help more generally to educate users, which may help long-term with the ID problem.
Are there external sources that talk about these taxa using terms that would be more accessible to non-scientists? In other words, are there existing common names that could be added to iNat? (Common names should not be invented for iNat, but if you know there are common names being used in other contexts you can add them or create a curation flag asking them to be added.)
For non-taxonomic groups (i.e., organisms that look similar but are not all related) there are a couple of ways you could represent this: traditional projects and observation fields.
Both of them have certain limitations, but depending on what you are trying to accomplish, might be useful. Both of them would require manually assigning observations to the project/observation field; it would not affect the ID on the observation.
For the first option, you would create a traditional project titled “yellow sponges”, a second traditional project titled “nori”, etc. and have your project curators add observations that fit these morphological groups to the corresponding projects. These individual projects could be linked by creating an umbrella project.
For the second option, you would create an observation field with a name like “general organism category” and a list of values for that observation field (yellow sponges, nori, etc.), and have project curators add this observation field with an appropriate value to observations in your project. Observation fields are somewhat more difficult to search and are not displayed as neatly as projects, but they could be implemented within your existing projects.