Can loose skin surgeries be safely performed at an underweight BMI, if the person plans on gaining weight after?

I recommend that my patients achieve their optimal weight loss and maintain it for six months prior to surgery.  You are correct that weight gain following surgery could impact your scars.  Schedule a consultation with a board certified plastic surgeon who can confirm the laxity of your skin as a result of your weight loss and recommend the path ahead for you to achieve the results you are seeking.

Congratulations on your weight loss!  To optimize your surgical outcome you should be weight stable for at least 6 months.   What I mean by this, is to continue your current weight loss and get to your goal. Once you get to your goal, maintain that weight for at least 6 months.  Once you have maintained your goal weight you will be a good surgical candidates.  Large weight fluctuations, either up or down are not ideal when performing body contouring procedures such as brachioplasty b/c it will affect your final cosmetic result.

You should schedule a consult with your local plastic surgery to go over all the options for body contouring after weight loss

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their
results.   The most important aspect is to find a surgeon you are
comfortable with. I recommend that you seek consultation with a qualified
board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Generally it's best to have the surgery near your ideal goal weight as weight gain after surgery may widen your scars.  Be sure to consult with an experienced board certified plastic surgeon.  Good luck! 

Surgery should be performed at your goal weight not below. You need to be nutritionally healthy with good protein and albumin levels not just low BMI for good recovery.