I just got a call from the vet! The results on Meeko's lump have come back (he had surgery to remove a lump on his back on Friday), and it's a benign cyst! An infundibular follicular cyst, to be specific, which is cured by excision (though it could recur) and won't cause him any serious health problems!
I am SO incredibly happy right now! I have been a wreck all weekend, thinking it could be malignant, and this is such a relief. THANK YOU all so much for your thoughts and vibes, you guys are the best!