Why not expand the hosta planting toward the back, coming as close to the stump as possible? They would hide it.

Put a small fern in the center of the stump--they'll grow and hide it until the stump disintegrates.

Paint it pink, and put the alliums in the middle.

Cut it flat, and use it as a base for sculpture. Something like an Alice in Wonderland mushroom, maybe.

Instead of trying to hide it, I'd definitely use an SEP field.

Yews will grow in part shade if you want something to hide it year-round. Rhododendron PJM is tough and also semi-evergreen.

Camouflage paint in dark brown (it's very non-reflective) could also help hide it, especially if you planted ivy or another ground cover that would cover it in time. It blends really well with most dirt, but is likely darker than your mulch so you'd probably want to test it on the back.

Cut the stump. Put a table on top of it, chairs around. Got it?
Suzi

If your not going to have it grinded down, understandable, because it is expensive... you should at least get some 1inch x 12 inch drill bits and drill holes strategically around the stump and then use stump rot potasium nitrate, urine (seriously..), manure, or other organic materials... in about 2 years the whole thing will be crumbling apart.

The more ideal way is to put kerosine in the holes and burn it, but it's too close to those other trees and probably your house for you to do that.

Here is a link that might be useful: Tree Stump Natural Removal