You didn`t say what the surface shape of the primary was going to be but it should probably be a paraboloid surface. As others mentioned, the negative lens would obsucre a fair bit of the primary to the secondary and that will cause differing problems of focusing the image and resulting in loss of contrast.
If you set the system up so that the flat mirror is near the focus (you don`t want it at the focus or you will be looking at dust on the primary) of the primary, you can use a positive lens a fair distance away as a relay lens and thus get the focus down beyond the primary, or better yet, put a flat at 45deg. and put the light out one of the arms of the DEC axis and thus get a EP location that doesn`t move when you reflect it again out the RA axis!