This sounds like the exact reason that UTFGrid was created! This site links to the tutorial that I used when learning UTFGrid, and it is solid.
Updated after your update:
MarkerCluster might have the look/feel you are going after, they basically paint a polygon onto the map layer. You can check the source here, and here's a relevant snippet:
_showCoverage: function (e) {
var map = this._map;
if (this._inZoomAnimation) {
return;
}
if (this._shownPolygon) {
map.removeLayer(this._shownPolygon);
}
if (e.layer.getChildCount() > 2 && e.layer !== this._spiderfied) {
this._shownPolygon = new L.Polygon(e.layer.getConvexHull(), this.options.polygonOptions);
map.addLayer(this._shownPolygon);
}
},