How to reverse Mel scale
The formulation for MEL frequency scale is logarithm scale based, it clubs(put together) the higher frequencies more closely(so less resolution), because logarithmic functions vary slowly for larger numbers(see the logarihmic functions plot).

So some function of exponential form will definitely work out if one wants to give more resolution to higher frequencies.  

so based on particular requirements one can define exponential scale for effect that looks like reverse of MEL scale.


Following is the filter bank I created using exponential function, similar to the formulation of MEL scale.  

$fnew$ $=$ $exp(f / 7000)$.

I think this is what you are looking for, more resolution for higher frequencies as compared to lower frequencies.