So, what happens when you have mild hearing loss? Mainly, your hearing threshold (should be around 0dB SPL at a few kHz) increases. But who listens to music at volumes around 0, or 10, or 20dB? At a more usual 60 to 100dB, the hearing threshold is not relevant anymore, so what's the problem at all?
Granted, there are other consequences as well, e.g. the threshold of pain decreases (you get more sensitive to high volume), and psychoacoustic masking mechanisms probably change a bit. But those things are way too complicated to be compensated by a simple EQ or the like. The brain, however, is much more powerful at correcting sensory shortcomings, it is constantly calibrating its sensors using known references, other senses etc., so as others pointed out, there is no objective point in equalizing or compensating anything. Of course you may personally like a certain EQ setting compared to another setting, but that is simply taste and should be found by subjective preference testing instead of medical hearing analysis.
Of course, stronger hearing loss is another thing. This really impairs your hearing at normal levels, but again, a simple EQ or just "turning it up" will hardly solve it. What you need in this case is - a hearing aid, with its sophisticated algorithms and acoustic parts, costing several 1000 $. Most hearing aids are however optimized for communication in difficult environments, not for recreating a somewhat natural, balanced perception of music. This is where a new product could shine, but it would still have to be based on all the medical, audiometric examinations, and use those sophisticated algorithms which have dozens of parameters which the audiologist iteratively adjusts to the individual needs. What would you guys think of such a "music hearing aid" headphone?