This is years late but perhaps others who come across this might find it helpful. I have a full right hip fusion and was able to have a vaginal birth without pain meds. I labored mostly in a jacuzzi tub at the hospital and when it came time for baby to come I gave birth in the bed on my side (left leg held by doula and right (fused side) on the bed.) So it's totally possible! Just talk with your doctor and ask what modifications might need to be made. For example, freedom of movement (e.g., not being tethered to the bed or a fetal monitor) is going to be important because you're not going to be able to get into all the laboring/coping positions someone without a fused hip can do. Good luck to all!
This may be too late for you, however others that are in the same situation, may find comfort in this article http://www.actaorthopaedica.be/acta/download/2008-6/08-Kirkos.pdf which explains post fusion after affects. 2 women that were involved in the follow up managed to conceive, and deliver vaginally 2 healthy children each. It shouldn't be a problem for you. I myself have a fused hip, and are considering getting pregnant too.
Your best bet is to ask your doctor what they want to do. It may hurt your hips and pelvis to give birth vaginally. It's extremely taxing on your body. You may feel better giving birth via c-section.
And you know what?! That doesn't make you less of a woman or mom if you give birth that way. The point is to bring a baby into this world as healthfully as possible! I know you know that and probably really want to give birth vaginally. Maybe your doctor will consent and let you try vaginally and if it is too much for you, then they could switch to c-sec.