if you truly mean a decorative SCARF joint rather than just a decorative inlay joint in veneer or of veneer, that would be a tall order. especially especially for a closed shape. a linear decorative scarf may be just achievable by an over lap and say a scrollsaw w/ tipped fence and a very thin blade or omni cutting abrasive wire on super high tension. you would have to preaccount for the scarf overlap length in the final pc. and your cutting would have to always be absolutely parallel-would need some kind of setup.

another way to get something similar would be to 'ditch' the rear profile all around but use a typical vertical cut. back scoop out the ditch sides just up to the top veneer to the usual 1:8 or 1:12 and fill w/ glass. at least then you'd get something similar structural to a scarf but with a way more easy typical vertical cut initially.

the difficulty about a scarf joint being a decorative joint, is that its very nature means an indistinct and fragile edge of the top material. if this property (a feather edge) is actually what you want at the edge of your decoration, what about sacrificing the total flat panel requirement of a usual scarf and let this feather edge raise and dip on the joint substrate so that the decoration actually exploits the character you want. in this case then, you could be more crude on the underlay material shape (as long as it is constant grain or colour or whatever background you wish) and you could use a little filler where you cannot see it under the decoration.

if you just mean veneer inlay, practice double cutting thru scrap veneer, or card or doorskin. use the thinnest scroll saw blades, or jig, or most likely if something like an exacto knife. as the veneer has grain and will/may break along these lines maybe back up w/ thin paper so that woodfractures can be repaired/controlled.

in some cases if you slope the blade very very slightly to the centre of the decoration you make like a steep scarf that the veneer may drop into. and seem to close the cutline right up. a scarved kerf??

-mick