While it would be nice to have object spacers as you described, that would then take up a slot in your furnishings totals for an effect you can currently achieve by glitching.
As in, set up a partition (or some object that can hold other objects, like a table or such also), put objects on it in the position you want the objects to be in in 3D space, then glitch move & remove the partition and the left over object are now "floating" where they were.
See my FC estate plot on
Faerie - Shirogane Ward 22 Plot 7, in the entrance hall with the torches and at the start of the museum next to the housing NPC where the guest book is, for some examples of object-float-glitching.
More info:
https://hgxiv.com/glitch-guide/
A better solution than what you described would be to allow for 3D placement of basically everything, assuming the housing engine can handle that.
That way, you do not have to use up an extra furnishings slot to achieve a stand-off of a wall mounted item.