Oh. My. God. Never mind. I had my 1 as 00001 in my calculation, instead of 11110. More details on the lookup table to follow to round out this discussion.
Perhaps I'm misunderstanding, but I worked out the table of byte codes in the 5-bit SPI protocol, and it looks like your Xs and 1/0s are exactly inverted. For mine, it came out like this:
X000XX00 0XX000XX 000XX000 XX000XX0 00XX000X
What am I missing?