I hope its still not too late to request this stuff from Autodesk.
looks like even 3ds Max has it :
Even Nucleus uses KDtree for most particle operations. Don't understand why its not exposed as a command/node for the users. It would be really useful to have it !
Using expression is not always efficient when dealing with high particle count & it is hard to get the indices of the particle that was previously emitted. The expression thus does a double loop over the particle age array to find the next youngest particle for each particle. Perhaps there could be a better way to write a particle field in C through the api, and fix the distances that way, As being api noob I could only rely on developers to have this integrated or available to us.