Air-sealing the eaves
The rafters form a plenum that runs over living space AND extends over the walls to the outside, outside the thermal envelope?  If so, I imagine that you are trying to extend the thermal break that the wall creates through this plenum?  If so, you can roll up fiberglass insulation, place in a plastic bag, then push into place right above the wall, where the rafters hang over to the outside.  Follow up with expanding foam to stop air flow.  I did this in my cape code with great results.

Something to be aware of though: Do you have soffit vents?  If so, air is supposed to be able to be pulled from these vents as air escapes from the attic ridge or gable vents.  In that case, you would need to preserve this air flow, which can be tough.  Some place small PVC pipes through this plenum, from the soffit area to the main attic above.  The trick it to move that air, while still maintaining good R-value on the interior ceiling.  PVC pipes usually require pulling off the fascia to push through, so I would insulate all the rafter bays but for one or two to allow air flow to the main attic.

Something I have learned dealing with an older house is that you can drastically improve things, without having to make them perfect.  I have lowered my electric bill from $230 to $160 per month in the last 7 years.