Consider halving the 90" span with a support post.
Make the 1x2 edge from maple or a similarly hard wood.
Use a flat lap/miter for the corner joint in lieu of the biskit.
Casey

Below is a system to figure sag.

Here is a link that might be useful: Shelf Sagulator calculation.

Thanks sombreuil mongrel and handymac for your responses!
I checked out the sag calculator, pretty nifty. Even though our shelf thickness is 3/4", I entered 1" thick shelves into the calculator given the addition of our 1x2 edging. Do you think that's an accurate reflection?
Based on the sagulator, it sounds like we may need support brackets every 30" - 36".
Can we notch the brackets so they fit over the 1x2 cleats?
Any others with their thoughts?

36" spans shouldn't be a problem. I build heavily loaded bookshelves at 32" with similar construction, supported by shelf clips at the ends. Rather than brackets, you might consider breaking your runs by using a 1x2 trim piece running floor to ceiling (or top shelf) every 36". You only need it at the front since you have a solid cleat nailed to the wall at the back.