That is a clever idea. If I am looking at it right, the rails are set so the pillow block slide part will extend past the uprights - motion not limited by the Z mount block.
It is not going to save you if the rails are not supported (sag will still be way too much for unsupported rail).
Maybe you can tell us a little more about exactly what rail / slide arrangement you have in mind. Those linear bearings vary a lot in capability.