Unfortunately, not really. It is possible to use custom patterns or even images for fill hatching, but those “repeat” in a way that I don’t think would be useful for what you describe. I think that kind of programmatic pixel-shading is probably out-of-scope for Bokeh. But cc @Ian_Thomas in case he has any thoughts to add.
The only sort of workaround I can think of might be to add the gradient image as an underlay to the entire plot, then add a “negative” opaque varea glyph to mask out the top part of the image. But I think that might run into issue with obscuring any grid lines, though from your example maybe you don’t have any (or alternatively maybe the render level of the varea can also be adjusted to sit “in between” the image underlay and the grids, etc.)