Is there any disadvantage to running an op-amp off asymmetrical voltage rails?
As long as the input pins or the output pin(s) of a typical rail-to-rail op amp are between the rail voltages, the "ground" reference is not actually locked to midway between the rails. (n.b. IIRC there are some op-amps which actually have a separate ground pin) 

To illustrate, let us consider a rail-to-rail input/output (RRIO) op-amp in a voltage follower configuration:

We supply the op-amp rails with +/- 9.15 Volts, and inject an AC signal with a DC bias of 7.5 Volts and a peak-to-peak AC amplitude of 3.3 Volts.

The output would traverse between +9.15 Volts and +5.85 Volts, which is within the operating range of this hypothetical op-amp. The rest of the operating range, from -9.15 Volts through to +5.85 Volts would be unused so long as the signal stayed within parameters.

The mid value, 7.5 Volts, can thus be treated as the "ground" for this signal.

In other words, the op-amp doesn't care about the symmetry of the rails, it doesn't know where "ground" is.



For a real op-amp rather than an ideal one, there may be some minimum headroom for the output swing, from a few millivolts to a couple of Volts. So, this needs to be accounted for in the strategy laid out in the question.

In addition, the MCU pins may not take kindly to a traversal below the MCU ground rail. Hence, a clipping diode to the ground rail, on the input side, would be a good idea.