I do not recommend using acepromazine for sheath cleaning. One of the possible side effects of ace in males is a condition called paraphimosis. This is a condition where the penis drops out of the prepuce, and enlarges to the point where the horse is unable to retract it due to swelling. Although it is more likely to occur in stallions, it has been reported to affect geldings in the same manner as well. This is a dangerous condition, as when the swelling progresses, circulation to the penis is progressively limited, and there is potential for loss of the penis, or loss of function.
If you obtained the ace from your Veterinarian and she/he is comfortable with the idea of prescribing you sedatives/tranquilizers, you might discuss using a small amount of xylaxine (rompun) for sheath cleaning. Xylaxine is a commonly used sedative that is very effective in causing the penis to drop. As I am unable to examine your horse, there is no possibility of my establishing what is called the patient/doctor relationship, which is legally required for me to prescribe prescription drugs. Please check with your vet for the proper dosages that will get the desired effect.
I hope this helps, and please let me know if you have any further questions.