I read of a builder who builds ceiling service cavities out of 2x4s perpendicular to the trusses.  The exterior walls that parallel the 2x4 s are built 3 1/2" taller than the other walls.  The interior walls and gable end walls support the 2x4s along with temporary bracing while OSB is glued and screwed to the topside of the 2x4's & taller exterior walls. The OSB panel joints are taped on the topside to complete the air barrier.  Then the trusses are placed atop the OSB  (the workmen are working on a comfortable floor, a complete plane of OSB).  After truss placement long screws hold the 2x4 s to the bottom chord of the trusses and the temporary braces are removed.   Has anyone here tried this or seen it done this way?  Is there a better way?

I'm a fan of service cavities and chases, but it seems like you could provide for utility access a whole lot easier than building a full-ceiling service cavity.

Another possible way of constructing a 2x4 ceiling service cavity could be: 1, Frame the exterior walls 3 1/2 inches taller than the interior walls. 2 install OSB atop the exterior walls, tying double walls together., leaving bout 3 feet (assuming 12" double wall) hanging over inside wall  3, install trusses.  4, Complete the ceiling air barrier by gluing and screwing OSB to the remaining interior area & tape joints.  5 Add 2x4s (long dimension vertical) below OSB using 6" long wood screws into bottom chords of trusses.  
Step 5 could either have the 2x4s parallel to or perpendicular to the truss chords and the 2x4s could be glued in addition to the long screws.  Which is preferable?
This method involves more work 'overhead' but doesn't require building temporary support (walls) which requires a fair bit of lumber that ends up scrapped but may require more long screws.  I suspect it's actually less labor intensive & therefore more economical to forgo the walking surface for truss setting.  Would 5" screws be adequate?

an easier way to make an airtight ceiling assembly is to staple a (intelligent) vapor retarder on the underside of the truss, tape that airtight layer to your airtight layer in your walls and then make a service cavity under this ceiling with 2x's.
Fast, easy and much lighter than mounting OSB overhead.

of course you need to connect this membrane over your wall if you airtight layer is on the exterior of your load bearing wall.

I really dont get the service cavity, even conventional prior to insulation its an open canvas. Seal away, easy no problem, why complicate matters in an uncomplicated situation. 2 part the wall junctures and skip the osb, blower door testing will confirm this.