Okay, I did some tests of the other spline filters via avisynth on the same clip. Methodology was to sequentially downscale and upscale the frame using the same filter for 5 repetitions, then look at the quality of the resulting image. I know this test isn't accurate or orthodox, but it has been done in the past and has really clued me on which of the splines were problematic in the past (as was the case with spline16w [wilbert] from an older thread). Also, sizes of compressed clips (x264) and png screenshots served as an even cruder means of comparing "sharpness."
Conclusion: Is there a problem with spline100resize? The image it produces is far too sharp (and filesize too big) compared to the other (working?) filters. All of the others (except spline16) were similar in visual appearance, as is expected from spline36+ in terms of the law of diminishing returns to how 'accurately' the interpolator can resize an image (yes, I used the two words together).
See the pics below. Remember, spline16w was verified to have a coefficient problem in the past, and it looks as if spline100 is following suit (though to a lesser extent). Of course, this anomalous problem could be reversed: if the splines are supposed to become increasingly sharp, then spline144 clearly isn't sharp enough! But I somehow doubt this to be the case.
Top: spline100, Bottom: spline144 (looks very similar to 36 and 64 too).
[edit]
Here's the tar with all test pics.