Willem:
Thanks for your question and I apologize for my slow reply. walking backwards uphill is actually a clever idea, although I’m not sure if carrying the pack on front or back really makes much difference. But, hey I’ve not done it and it sounds like you might have played around with it.
We do use backward walking on inclined treadmills and dragging sleds to strengthen the VMO (vastus medialis oblique) muscle. That’s the teardrop shaped muscle just above your knee on the inside of your thigh. It gets heavily involved in descending when you knee goes out over your toes. To train it you do need to step well back (uphill) so that the toe of the upper foot is behind the downhill foot and the knee has a significant bend in it.
Descending scree is always going to be tough due to the instability and balance issues and I don’t think this backwards walking should be tried on scree. I think you will end up falling a lot. 🤦♂️
I hope this helps.
Scott