Mounting a mini split head near the floor is not practical. if the ceiling is 8 feet high mounting near the ceiling will not be a problem.

They make floor mount units. Your right that you cannot mount the wall units near the floor. They wouldn't work properly.

https://www.fujitsu-general.com/us/products/split/floor/index.html

So it sounds like it would not be good to put a wall unit near the floor, but if I can get a floor-mount unit, it will be more efficient than a wall-mount unit properly mounted high on the wall?

For heating yes.

The floor mount is for heating only. What will you do for cooling?

This is to go in a walk-out basement area used as living space. It never gets hot there. We're thinking about cooling the rest of the house (above) by blowing cool air up from the lower level, using the ducts. Sound reasonable?

For an underground basement the floor mounted one is a good application. The heating load is small and the cooling load is usually zero. Walk out basements are more exposed but it should still work well.

Pulling the cool air from the basement is not going to be sufficient to keep the upper floors cool. It will help but it is not going to perform like an AC system.

If you need heating and cooling in a floor mount they also have these. I didn't realize I had linked to a heating only unit.

https://www.fujitsu-general.com/us/products/split/floor-ceiling/index.html

When we bought our AC (ceiling mount) we also got the heat capability. The cost over the cooling only was negligible. The same might apply to the heating only vs heating and AC.

Your plan to pump the air up from the basement will not work as the replacement air in the basement will be the hot outdoor air or the hot house air and soon you would be pumping hot air up.

You don't say where you are or your mix of heating/cooling loads and size of house upstairs or if you have other heat source, but it may make a lot more sense to simply put another minisplit on an upper level for cooling when needed - will also be useful for heat. And likely will be a lot more effective than trying to duct and push air upstairs.

Another reason to consider a separate unit upstairs: equally important part of ac is humidity control, and just circulating air from basement up (and back, it has to come from somewhere) won't remove humidity.

Plus potential mold issues. (another reason to have a unit capable of cooling downstairs too - may be useful for dehumidifying in peak seasons, if that's an issue in your case)

Thnx Armoured, good thoughts. Do all mini-splits dehumidify, or just some? Do I need to check that on one that I am considering?

All work to air condition by removing humidity, and I think any newer ones should have a dehumidify mode as well (prioritizes dehumidify over dropping temperature). Do check of course. If you think you will need to heat and dehumidify, that may need a special mode (basically to switch modes occasionally), and I don't know about that. That may be something worth looking into if the basement tends to damp.