At Simon's suggestion, I compiled a list of all the rules that have been used in the game so far
All rules take the form: "A koan has the Buddha-nature if and if only..."
For rules that refer to vowels/consonants, it is noted whether Y counts as a vowel for this particular rule (although in three cases, the round ended without resolving this ambiguity!)
1. (Simon): its letters are not all different
2 (Insane Steve): at least half its letters are vowels (Y is consonant)
3 (geoo): all letters have either mirror or rotational symmetry
4 (ccexplore): it contains an odd number of consonants (Y not tested)
5 (möbius): exactly half its letters are vowels (Y not tested)
6 (Minim): each consonant occurs at most once (Y not tested)
7 (Simon): its first letter is alphabetically earlier than all its other letters
8 (Proxima): its length is greater than or equal to alphabetic position of its first letter
9 (ccexplore): it contains at least one letter from "AKHTBN" and letters from that string occur only in that order
10 (Clam): it contains two letters that are consecutive in the alphabet
11 (Nepster): exactly half its letters are from the first half of the alphabet
12 (Proxima): its first and last letters are the same or their alphabetic positions differ by a multiple of 4
13 (Ramon): it contains the letter whose alphabetic position equals its length
14 (Nepster): when arranged alphabetically, its last letter is a vowel (Y is consonant)
15 (Minim): the sum of alphabetic positions of its letters is prime
16 (Proxima): its length is square or triangular, but not both
17 (Clam): its third letter is the same as its third letter when arranged alphabetically
18 (namida): it has at least three letters, and at least half its letters have (alphabetic position - 1) a multiple of the length, with the first letter counting twice if it is a multiple
19 (Proxima): for each n, the nth vowel either does not appear or appears n times (Y is vowel)
20 (namida): every pair of adjacent letters are either the same or from opposite halves of the alphabet
21 (Proxima): it contains an odd number of letters with exactly one vertical line segment
22 (Simon): its first letter is the same as or alphabetically earlier than all its other letters
