Hello Metalreefer,
From Barcelona (Spain) i just discovered your thread, and i can only tell you; GO AHEAD !!!!!!
Sorry for the English version (Google translate).
I'm going to try to help you with your great project (this one), or the one you have planned for the future.
I am a happy retiree, with a lot of time for my hobbies, and this allows me to read Forums as interesting as R2R.
Many years ago i finished my studies in Biology, i was lucky enough to work for a season at the Barcelona ZOO, in the collection of Poisonous Snakes (ATROX) in the Terrarium of the zoo.
Then all my life I have kept Vivariums, Marine and Fresh Aquariums, Terrariums, etc ...... as a hobbyist.
I currently have a project, for next summer, if the Covid allows it, very similar to yours; a biotope of the coastal zone of the Mediterranean Sea, with a submerged zone (Posidonia meadow), a tidal zone and a terrestrial aerial zone, all with native species of the Catalan coast. Aquatic area volume 600 lts.
I am enclosing a photo with a lousy handmade scheme that could suit your idea. It's a profile view of a terrarium like yours.
It is based on a standard partition, with a glass partition, between the two aquatic areas, the marine and the freshwater that you want to set up.
You can design the glass partition as you want to suit your project; straight, with different angles, at the height that suits you, in the depth you need, etc .....
At the bottom of the terrarium, and as a support for the waterfall, a false bottom (F.B.) is installed with an angle of inclination (alpha) that will allow the water from the cascade to flow like a curtain and not splash the marine area. The flow of this waterfall must be such as to allow this situation. Install a regulating valve at the outlet of the inner filter (I.F.) of the fresh zone. From this you launch a tube to the top of the waterfall, which will have a horizontal perforated tube to create several lines of water fall, and better distribute the curtain of water from the waterfall. All this is the key to the proper functioning of the waterfall.
At the top of the waterfall, or wherever you want, you can install the tank for the mist creation pump (MP), inside the terrarium or outside (in / out), making a hole in the glass of the terrarium if it is outside. In this case, the tank must have an access cover for filling water, and maintenance ......
The outline photo of the waterfall (Waterfall profile) and the rest of the background decoration (Rocs profile) are included in the diagram photo.
The fresh water zone (Fresh) will have a standard heater with thermostat.
It is important that you have access (Hand access) to this area for proper maintenance. You can install an overflow hole (W. Exces) of the duct water level, to avoid contact between the fresh and marine zones. It is not essential.
In the marine area (Salted) I propose the two standard perforations for overflow (To filter) and return (Filter return) of water to the filtration system.
This can be a sump, a bottle filter, etc ..... If it is a sump you can include all the equipment you think is necessary.
If you don't want to pierce the glass you can install an internal filter (not included in the schema) and a heater (T.H.S.).
With the rocks for the aquascape of the marine area you can hide the installed equipment, always with easy access for maintenance and cleaning.
From my experience in caring for poisonous snakes, reef aquariums, vivariums for Dendrobates, etc ..... you must foresee 99.99% of possible situations and incidents in the operation of the system.
Putting your hand once installed and running has its problems and risks ........
Take time to simulate all the situations that occur to you !!!!.
Find all the information you can about the compatibility of the plants you want to introduce in the aerial zone, with respect to the evaporation that will occur in the marine zone and that will drag a "salty" environment.
In your big country you have countless ecosystems that are related to your project.
For any questions, queries or information, let me know.
I am very interested in your project.
Greetings from Barcelona, Catalunya, Spain, Europe, World .......
P.D.: I don't know how to add the photo of the scheme !. help !