I'm not sure why you need a viewport, but you can probably attach a CanvasItemMaterial on the sprite, with a custom shader like this:
// This shader will make an item more transparent the darker it is.
// Adjust the ramp to choose how dark it must be to become transparent.
uniform float ramp = 1.0;
// Get texture pixel color
vec4 col = tex(TEXTURE, UV);
// Compute grey scale taking luminosity of colors into account
float gs = col.r*0.299 + col.g*0.587 + col.b*0.114;
// Compute modified transparency
float alpha = col.a * clamp(gs*ramp, 0.0, 1.0);
// Assign final color
COLOR.rgba = vec4(col.r, col.g, col.b, alpha);
You could probably make your thing transparent in the first place, but I don't know which are your requirements/constraints.
Edit: if your thing is actually transparent, you could simply try to enable
transparent BG in the inspector when selecting your viewport.