Allowing custom function for merge as_of "direction"
This is an idea:

The suggestion is to allow the user to define a function specifying how they want the merge "on" to be done.
For example, the best I can do now with the predefined "nearest" is to match the "nearest" value on one column, but let's say I wanted to match the "nearest value" for a function of the values from two columns e.g. distance from x and y coordinates this is not possible.

I think my only other option would be the much less efficient method of iterating through my two dataframes.