That is a great building-technique. I especially like the "rocksolid-version" with the expanding epoxy foam.
We did quite some experiments with PU-foam as core material at some moulded parts (for example building moulded ailerons). I like it wherever weight is not a primary factor and "ease of building" is the dominant criterium. At parts with no interior structure like shearwebs or subspars it works incredibly well.
For us it took a little while until we figured out how to build good parts out of our moulds.
I also am yearning for some low density PU-foam with longer (lets say at least 5min.) potlife but found nothing up to now.
Therefore we needed to cope with this crazy fast stuff and found a - lets call it: "three steps closing procedure".
We apply thickened epoxy (I use 2 portions of erosil and 1 portion of flox) to the leading edge on both halves of the trimmed parts in the molds. It's about the same thing you'd do when closing a hollow moulded structure.
Then we wait for the epoxy to reach its gel-time. That's about 2 times its potlife but tests should give you the right moment. It's perfect when the "epoxy-worm" barely sticks to your glove anymore but still is soft enough to fully close the mould.
We use a wide brush (1.5 - 2 inch) and apply fresh epoxy to the vacuumed and cured skins. This epoxy is mixed with about 3% of foaming agent.
It helps especially on the trailing edges but as well with bonding the PU-foam to the wingskins. I was researching about combining PU and EP systems and found nothing accurate, so I gave it a try ... and it worked.
Pour and brush in the PU-foam like you used to do it, just be a little more careful at the leading edge.
With this technique the leading edges are solid. The expanding PU-foam has no real chance to push the epoxy onto the parting board because it is already to thick to move.
Give it a try! ...and keep up the good work!