I need a knot to attach the self blocking non-releasable part of the pulley used to my slackline setup with a small piece of cord. Therefore the knot should be secure for high loads and against accidental releasing. On the other hand it needs to be releasable safely (without fingers getting crushed) while the slackline is still under tension. The release does not have to be controlled however, it is allowed to release fully immediately.
I try to find a suitable minimal slackline setup and the method explained here has too much friction for my use case. So I went for a slightly less minimal variant where I use a cord (static 6mm) to setup a pulley. So I needed a one-directional blocking knot/device for this cord instead of the flat webbing. All methods I know from climbing are not releasable under tension (except Grigri which does not block with the 6mm at all and is not really minimal).
If you happen to know a knot that blocks in one direction, is releasable under tension and is "minimal" (i.e. using two general purpose climbing carabiners would be fine, using a dedicated device not), that would also solve my problem.
As always when I try to explain something, it gets messy and hard to understand. I hope the following schematic drawing can shed some light. Please excuse my poor writing and drawing skills.