Normally a dermatologist is the one that does those biopsy's. Mine did mine when we were trying to get me DX'd. I understand your frustration with your doctor and the whole business of getting the referral and such. However, might I caution you, sitting out in the sun like you plan to do, may bring on a lot more than a rash and what you expect.
I've had lupus for 26 years now, it took my doctors 13 years to say yep I have and start treating me. Lupus can mild or it can be very severe. As I'm sure you've seen one of our moderators just received a kidney transplant on Tuesday. We have some folks here who are in what I call a remission and are medication free.
I'd hate to see you spend all the time in the sun to get the rash up in full bloom, and not only that happens, but you put yourself into kidney failure, or something happens to your lungs, or heart. Lupus can affect any organ in your body, plus your joint. I agree fully, that you need to see a Rheumy ASAP, but please be careful this weekend with your plan.
If you start to feel really bad or strange go to the nearest Urgent Care or ER for help. Good luck and try not to get to down over this. Lupus is a tough one to DX and treat times. Hang in there and know we're here for you if need us.
DX- RA, Lupus, Fibro, LGL Leukemia, Obstructive & Central Sleep Apnea, Pulmonary Hypertension, Bells Palsy, Gastroparsis, Blood Clots, Glaucoma, Chronic Pericarditis & Pleurisy, Severe Anemia. Way to many medications to list.