I've googled around quite a bit but couldn't find a distinct name for this notation. Should I conclude that this is some personal invention by Robert Coinel (from which this image is taken here)?
These thick horizontal lines are referred to as "extenders" in Behind Bars - The Definitive Guide to Music Notation (p. 632).
Gardner Read's term for thick horizontal lines like that, as well as similar innovations such as the posted example's eights-becoming-sixteenths beaming, is time notation (p. 72, Music notation: a manual of modern practice, 2nd ed., 1969). He illustrates this with excerpts from Earle Brown's 1959 Hodograph I and Luciano Berio's 1960 Circles.
Brown himself calls this notation proportional.
LilyPond calls them Duration lines. I have no idea whether this is a standard term.