If an object or person in a photograph or film is "en amorce" it is certainly in the foreground, but if that were all it meant they would presumably just say "premier plan" (or perhaps "gros plan": extreme close-up). The point is that the object is at the side of the frame and only part of it is seen; it is partly out of frame. The second photo in Phil's source, which suggests that it means framing the subject between foreground objects, is actually misleading here. The first photo, "Philippe de Vaucelles est en amorce droite cadre", is a perfect example; the person mentioned has his back to the camera and only the left-hand side of him is in the shot.

The best definition I've found of "en amorce" is the following, which has the virtue of explaining why the word "amorce" is used for it:

"En amorce, se dit d'un personnage ou d'un objet cadré en bord de champ, au tout premier plan, de telle sorte que sa partie vue soit clairement interprétée comme « amorçant » sa partie non vue."
http://www.larousse.fr/archives/cinema/page/37

So a "prise de vue avec plan en amorce" is an over-the-shoulder shot: the person over whose shoulder we are seeing the shot is "en amorce", in the foreground and partly out of frame.
http://termbank.com/en/french-english
Here it's called "plan avec en amorce l'épaule d'un personnage":
/prise%20de%20vue%20avec%20plan%20en%20amorce
https://books.google.es/books?id=zjefDAAAQBAJ&pg=PT100&lpg=P...

Here "les corps parcellisés en amorce" is rendered as "fragmented bodies at the border of the frame":
https://books.google.es/books?id=i7q5FlzEeDUC&pg=PA31&lpg=PA...

You would think there must be a short expression for this in English, but I can't find one. You could say "in the foreground, partly out of frame" (or "partly in frame", for that matter), but it's a bit of a mouthful, and I think the important part is "partly out of frame".

I think "amorce" in your second passage has the same meaning, and is just an abbreviated form of "en amorce".

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Note added at 7 hrs (2017-03-04 23:32:22 GMT)
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Some excellent examples of characters "en amorce" here, both "flous" and "nets" (in soft focus/blurred and in sharp focus):
http://www.arretsurimages.net/breves/2014-09-15/Net-Flou-id1...



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Note added at 19 hrs (2017-03-05 11:23:06 GMT)
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It is significant, I think, that in the source I've just cited they repeatedly say "en arrière-plan" versus "en amorce au premier plan". I conclude from this that although people/objects "en amorce" are indeed in the foreground, the primary meaning of the expression itself is something else, namely being partly out of shot/frame, otherwise there would be no need to include "au premier plan"; it would be a tautology.

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Note added at 20 hrs (2017-03-05 12:24:57 GMT)
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And pursuing that point at little further, since it clearly says that in these photos the product is to be "positionné en amorce", and the product must obviously be the subject of the photo, it seems likely that they envisage the product "en amorce" being "net" and the background "flou". A shot like that would be quite effective.