Lay sonus faber on the side?

As long as the finish is protected, this shouldn’t damage speakers. 
 

But as mentioned above… it will completely screw up the sound. I used to own a pair of Olympica 3. They need distance behind and on both sides… and definitely be upright. The external cabinets are purposefully designed in all aspects. 
 

I would trade them in for some cheap box speakers made to be close to a wall… although being enclosed we make them less than optimal.
 

Say it ain't so.  SF may file a grievance against your architect. 

Terrible idea

Sell them and replace them with something better suited to your new use case.  If you are having built-in cabinets constructed, get speakers that will perform properly in that context.  MoFi Pointsource or KEF, Fyne, or Tannoy all make “coincident” drivers that radiate the same polar power regardless of orientation.  Without a floor plan and cabinet drawing, that’s the best I can do.  One thing’s for sure…laying Olympica IIIs on their sides is lunacy.

No, not at all.  In fact, you can even play them that way, although they won’t sound as pretty.😀