I found a way to speed up the brute force approach though. If you can use external Python modules:

from scipy import spatial

tree = spatial.KDTree(t_list) # t_list is a list of pre-calculated point coordinate tuples on the cubic curve
distance, index = tree.query(pt) # pt is the given point coordinate tuple
t_for_pt = float(index) / (len(tree.data) - 1)

t_list must be constructed in a way that the first entry is for t = 0 and the last entry for t = 1. It doesn’t matter how many points for increments of t you calculate, as long as the index in t_list is proportional to t. Smaller increments of t increase the precision of the result, but the list takes longer to build.

It seems the problem is that some of your layers are called 1, 2, 3. RoboFont doesn’t seem to like that. In the console, there’s this message:

RoboFont[14862]: -[OC_PythonNumber isEqualToString:]: unrecognized selector sent to instance 0x7fad4ce63150

Rename your layers in the inspector window, and the glyph window will open again.

Same thing happened on my machine (10.9) when updating the FontTools in the site-packages folder. It seems some external modules are used by RoboFont when they are present, which causes some confusion in this case.

You could either delete the FontTools module in /Library/Python/2.7/site-packages or update the RoboFont-internal FontTools module (/Applications/RoboFont.app/Contents/Resources/lib/python2.7/fontTools) to the same version as in site-packages.

Setting a kern pair to zero in fact deletes it: https://github.com/robofab-developers/robofab/blob/master/Lib/robofab/objects/objectsBase.py#L3119-L3121

I can think of at least one case in which you would want to differentiate between a value of 0 and a nonexisting kern value, which is when the kerning pair is an exception to a class pair.

Something along these lines:

component_names = [component.baseGlyph for component in f[i].components]
if not "K" in component_names:
    f[i].appendComponent("K")