I think there is no access.
However, if you look at a group, you can see it is a point object. if you do CSTO on a group, you see that oyui get a polygon object with points. That has a bunch of channel vertex maps, and vertex color maps tags on it.
Float vertex maps are then for values, like lifetime.
Color Vertex maps are for anything requiring a vector such as angularvelocity.
not sure if these are just constructed when you hit csto, or if they are virtually present on a group(not tested)
the polygon object points, are the positions.
These maps can perhaps be used to interpret particles, but I don't see any way to create or change anything, or create any kind of modifiers specifically for them. it is a blank sheet. Let me know if you find anything useful
best
Paul