Ok...I think we have figured out my problem and it's not shoes that are too narrow (I picked-up a pair of Specialized mtn. shoes on eBay for a good price...strangely, they are narrower than my Sidis. Did a short ride with those and had numb feet the entire way. ).

I started Googling for info. re: sore outer arch area from cycling and MANY of the threads I found pointed to supination issues (interesting, as I am a mild overpronator when I run). The solution was placing a wedge between the cleat and shoe, on the medial edge.

I ordered a pair of SPD mtn. cleats with a 1º wedge built-in (plastic wedges apparently do not work well with the small platform of mtn. cleats, otherwise). While I wait for those to arrive DH made a small, thin, angled piece of aluminum with a hole drilled in the center to stick under the medial bolt.

Today I did a short ride, but it was pretty apparent immediately that pressure was redirected to the center of the ball of my foot, instead of under my baby toes. Sweet.