In answer to my own question, I've written a program to "vectorize" an ArcInfo Grid ASCII files as an ESRI shapefile with a single layer containing oriented polygonal grid squares centered at the points of the grid, with an attribute value equal to the value at the coordinates of the centroid. The program (still under development) is [available on GitHub][1]. An [example of a map][2] produced with the program is on CartoDB (it might not be for long). Perhaps one could have done this in GRASS--I prefer to attempt to understand the file formats.


  [1]: https://github.com/flengyel/ArcInfoGrid2Shapefile
  [2]: http://cdb.io/YdKps7