numeric_modifier() for block rate?

I'm attempting to write an ASH function that will return the best familiar to use for a given modifier, similar to the one that exists in zlib, but using ASH's numeric_modifier() function instead of a map file with semi-arbitrary values. Mostly, I'm writing it just to see if I can. While working on it, I found that the block rate of familiars such as the Levitating Potato, Untamed Turtle, etc, is not available via numeric_modifier()? Is this data available somewhere else or some other way without hard-coding it into the function? Or did I overlook something obvious?

It isn't currently available in Mafia, and from looking at the wiki is pretty poorly spaded for many of the block familiars.