Mo -
I seem to remember that there is a scarf in the loft @ Heartwood - not sure if it is part of a truss though...
Per your example above - a few concerns - and this is totally theoretical as I've never done this type of joinery... and would have to run it by an engineer if it was mission critical.
1. The hole in your through tenon limits how effective the KP can be as it goes into tension. You'd have to be sure you have enough relish on the tenon before you get to decoration.
2. The iron bands might want to be 2 u-shaped segments that are wedged together to allow for tightening (or use RF dried or reclaimed)
3. I'd move the struts up away from the tie... while I like the look of struts coming into the corner of a truss - if the rafters bend and the struts go to compression they push down on the tie - further stressing the KP. By moving them up 12" or so (dependent on joinery and how much relish you need between the through tenon and the mortises for the struts) you eliminate this variable. If the struts are not mortised but only deeply housed you can probably get away with 3-6" between bearing on strut and top of tie (depending on loads).
4. You might consider doing a splayed scarf with undersquinted ends - this will help key each end into the other.
5. Pegs could be a problem as Ken described - maybe wedges under the tie through the tenon to pull it up to the bottom of the KP? But this affects relish, etc...
6. In tension - longer is probably better.
There is information here on a 64' span that was scarfed for 'heavy tension loads":
http://www.tfguild.org/joinery/part6.pdfLook for the 'stop splayed with multiple tables'... although I see no example of a KP coming through the scarf. This may be due to wanting the scarf to act in as few directions as possible - as in a scarfed plate placing the scarf in the sweet spot away from the top of the post and high shear area and where the bending curve is not yet too great.