There are several reasons why plastic surgeons shy away from buttock lifts. The first is that in most cases the belt line lift procedures will lift and contour the buttock and the side of the thigh. We get all that we need out of the single procedure.
The second reason is that additional surgery or secondary procedures as we call them are indeed more difficult. The results may not be as predictable or satisfactory to our patients, and therefor require careful consideration and understanding. No patient or surgeon will likely take them on without caution.
Secondary buttock thigh lift involves the lower incision you mention across the lower fold of the buttock. The risk can include dents and depressions, scar asymmetry, a flattened buttock, and scar widening or descent or a pulling lower of the scar. In the healing period patients cannot sit fully for up to four weeks to avoid tension on the suture line. Despite all the drawbacks the procedure has been gratifying to patients.
I suggest you seek a plastic surgeon with experience and excellent judgement who will be with you throughout. You should understand your procedure well to avoid disappointments. It is an accepted procedure though, and might be what you are looking for.
Best of luck.