Hi Erns,
The planned goal for interior decorating of the pond and pool room is to be very clean. Water, decorations, everything. The focus on the koi. I have always loved water plants, but have not had any for years because the koi also love them and of the mess just from the soil and such. you mention. That being said, I have toyed with the notion of some plants in the catch basin area. I will probably have some if I can keep it clean. The notion of "soil-less" plants, ala what my friend Ray Armstrong has done is the direction I would be looking. I am also concerned about humidity, and plants might add to that.
No plants in the top. The area is somewhat small at 6' spillover edge, by 5' by 5' by 3' deep, and with 6000 gph going through there, I think not so calm. My aim in that area is to have the water look like it is "welling up", and spilling over calmly as possible. Decoration in that area currently planned to be a japanese lantern of stone, or some such thing. Maybe a Bonsai if enough sunlight.
Just got a call from the contractor. He can't get the concrete for the filter room floor til Monday, so now we are delayed on that, and the framing to start the following Monday. I will be out of town all that week, but the show will go on without me . Oh well.