I remember seeing someone here was/is using floorstanders placed sideways for rear surrounds. If that can be done, no reason why it cannot be done for the Mains.
That said, the floorstanders are optimized for vertical orientation. The vertical line of speakers creates a lobe of sound in front of it, for which the timber characteristics vary depending on if you are on axis or how far off it. As long as you mount the speakers such that your seating is on axis, it will minimize the loss of sound quality.
Now the on-axis vs off-axis property helps greatly in creating a soundstage, that is, the impression of there being sound sources outside the speaker and spread out in in the listening area. That is why placement and toe-in become important items to consider when judging speakers. With a horizontal orientation, this lobing effect will not be as expected (due to speaker use against design considerations). Therefore, it will impact your soundstage in an unpredictable manner.
All that said, give it a shot. Try the vertical orientation and get the best possible sound. Now change the orientation to horizontal and see if you can recreate the sound without any perceived deficiencies. If it works for you, why not.