I'm looking for stuff that would be in the vein of Forest's Untitled (second off Обрекая Надежду На Вечность) or Winter Howl off selftitled tape.
http://www.youtube.com/watch?v=-T58EE6ncvw
http://www.youtube.com/watch?v=NCvwMsFFwqQ

The core of what I'm looking for are long, repetetive non-metal songs, still using (black metal inspired) guitar playing, possibly drums and keyboards. And preferably, the chant mixed with whisper (possibly background) vocals that don't make you lose impression you're listening to instrumental music, even if they play a huge role in creating the ambience. It can be fully instrumental too, like the Branikald ones.

I'd have to suggest you're looking for stuff like Aakon Keetreh's 1997 demo, Soufferance's 2006 demo, Thorns' 1991 demo and Beyond Light's 2009 demo.

To call those tracks in the OP "ambient" is a bit misleading. But anyway, these should fit the bill perfectly:

Wigrid
http://www.youtube.com/watch?v=w6jmGHnQlso
http://www.youtube.com/watch?v=WGnUArMPOyY


And maybe these, from my old BM project Church Ruins:
http://churchruins.bandcamp.com/track/dust-and-ashes
http://churchruins.bandcamp.com/track/church-ruins