Stevenk,
I thought that this is the kind of frame you have in mind. Only thing I would add, at least in the gable walls where they are not in the way, are braces on the post underneath the gird and the ridge beam down onto the tie beams. Otherwise you do rely on the SIPs somewhat. There is a picture on the previous page in the book mentioned where it is done that way. Also like the note underneath the picture: "Best frames are often ........"
In your case the loads from the rafters, which will have to be sized so they can span and carry the loads, will be transfered the gird and ridge beam. From there it will get transfered down through the posts onto the foundation.
If you put the rafters too far apart they will actually have to be very heavy. I prefer this kind of roof construction myself. Besides the aesthetic aspect it is also easier and faster to cut then the typical bend and purlin approach. It is also easier to add roof overhangs at eave and gabel with out having to rely on SIPs. It makes the house more attrative and you do not have to box them in like the SIPs.
And as for lateral forces, since they are no braces in direction of the rafters. Since the rafters are fastened to the beams you end up with a rigid triangle formed by rafters and girds (which are held together by the floor joists) that can and will resist any force it has to withstand.
So this is a sound frame by itself that does not rely on SIPs for strength.
But just like any other frame that is covered with SIPs they will add additional strength thus stiffen the frame even more.
I'd say go ahead with your idea and post some pics as you go along.
