Am I still able to undergo a BBL if I have an autoimmune disease?

This will depend upon the severity of the MCTD. In lupus specifically, certain organ systems of the body may be affected. Any organ damage that the lupus has caused as well as the medications that you take must be evaluated carefully by your rheumatologist before even considering the BBL. You will need medical clearance from your rheumatologist as this disease has a very variable presentation and course.
If you obtain clearance from your rheumatologist, the medications taken will have to evaluated carefully as well to optimize healing and prevent possible wound breakdown or other complications.

In order to get good answer to this question you should have your internal medicine or immunology specialist involved in evaluation. Once you have that cleared, your plastic surgeon should communicate with that specialist and evaluate risks involved. Good luck.

Dear Sof27,
it is hard to tell for sure without an examination. Your condition should be under control and you will need a clearance prior to surgery. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery