Hey Don,
I have a bit of experience roofing and just this summer I did did a roof where the inside ceiling was attached directly to the roof rafters and isulation filled the entire gap between the ceilng and the planking (or sheathing), thus no air circulation. Combine this with inadequate ventilation at the ridge and the consequence was complete rotting of a good part of the sheathing at the top of the roof, some of the rafters were in pretty tough shape too.
I spoke to a very reputable roofer in my area and he recommended solving the problem in just the way you mentioned. I took off all the old sheathing, and nailed 2x2's all the way along all of the rafters from the edge to the ridge and then applied all new planking. There are some products that you can get to put in the hole made by the 2x2 spacers so that bugs and such don't get in them. You might also want to consider venting your ridge.
This was a stick frame home but I hope the situation was similar enough to your own to be of some help.
Drop me a line if you have any questions
