Tom...
the basic flaw with the concept isn't so much the jointing method, as the dynamics of the materials; I'll try to explain.
You've been around here long enough to have read a few times that natural wood is a dynamic material; it expands and contracts seasonally in reaction to seasonal changes in humidity. That expansion / contraction is predominantly tangental i.e. it moves a lot more across the grain that it will move along the grain. Now.. try to apply that to your top design..
As I understand it, you want to surround a pine top with wide mahogany edging on all 4 sides, mitering the mahogany at the corners... it's an idea I've had myself more than once...
Now... try to model that tangental movement with your design. The long grain to long grain joints are the strongest, and most stable too as there shouldn't be any dynamic conflicts. However, the long grain to end grain pieces at either end of the top will have a much harder time of it; your jointing will need to allow for some movement, but it's technically possible to do (breadboard end being an example). The worst of the trouble is at the mitred corners. The boards capping the end grain will hardly be moving at all, while their mating halves will be expanding and contracting quite freely. That is, until you make the glue joints at the corners. In time, the seasonal movement will trash the joints due to the unequal rates of expansion / contraction; the joints will weaken through fatigue, leading to their eventual failure.
However... it's not all doom and gloom...
IF.... you concede the point about the mitres, but figure that the look of the wrap around mahogany is more important than how it's joined at the corners, you can make it work by....(you guessed it) using breadboard ends. They'd work pretty damn close to how you've drawn them, although I'd be inclined to use just 5 M&T's across the width of the top (centre+each end of the pine section & 1 in each of the side pieces). As Frank said, lock each M&T with a dowel through each tenon; you can glue the central dowel in place to centralize any expansion, but you must slot all of the outer tenons to allow them to "float" across the dowels. Remember to keep your slots tight in the long grain axis (barely wider than the dowel) but loose in the cross grain axis (to let them float freely). You can lock the outer dowels in place by driving them most of the way through the joint, adding a dab of glue to the holes just prior to driving the dowels home.
As your sketch (nice sketch btw) suggests, you can reinforce the whole with additional tongues, but don't glue them.
For the record, I'd still edge the lot with razor wire to deter errant backsides.. :wink:
