Automatically applies Preset files to assets in the same directory. Also adds labels present on the Preset file to those assets.

PresetImportPerFolder.cs

using System.IO;
using UnityEditor;
using UnityEditor.Presets;
using System.Linq;

// This script automatically applies Presets to assets when they are imported into a folder containing a Preset.
// It also adds any labels on the Preset file to the asset.
public class PresetImportPerFolder : AssetPostprocessor {
	void OnPreprocessAsset() {
		// Make sure we are applying presets the first time an asset is imported.
		if (assetImporter.importSettingsMissing) {
			// Get the current imported asset folder.
			var path = Path.GetDirectoryName(assetPath);
			if(path == "ProjectSettings") return;
			while (!string.IsNullOrEmpty(path)) {
				// Find all Preset assets in this folder.
				var presetGuids = AssetDatabase.FindAssets("t:Preset", new[] { path });
				foreach (var presetGuid in presetGuids) {
					// Make sure we are not testing Presets in a subfolder.
					string presetPath = AssetDatabase.GUIDToAssetPath(presetGuid);
					if (Path.GetDirectoryName(presetPath) == path) {
						// Load the Preset and try to apply it to the importer.
						var preset = AssetDatabase.LoadAssetAtPath<Preset>(presetPath);
						if (preset.ApplyTo(assetImporter)) {
							return;
						}
					}
				}

				// Try again in the parent folder.
				path = Path.GetDirectoryName(path);
			}
		}
	}

	// Also add Labels on the preset
	static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths) {
		foreach (string assetPath in importedAssets) {
			TryApplyPresetLabelsToAsset(assetPath);
		}
	}
	
	static void TryApplyPresetLabelsToAsset(string assetPath) {
		// Get the current imported asset folder.
		var path = Path.GetDirectoryName(assetPath);
		if(path == "ProjectSettings") return;
		while (!string.IsNullOrEmpty(path)) {
			// Find all Preset assets in this folder.
			var presetGuids = AssetDatabase.FindAssets("t:Preset", new[] { path });
			foreach (var presetGuid in presetGuids) {
				// Make sure we are not testing Presets in a subfolder.
				string presetPath = AssetDatabase.GUIDToAssetPath(presetGuid);
				if (Path.GetDirectoryName(presetPath) == path) {
					// Load the asset
					var asset = AssetDatabase.LoadAssetAtPath<UnityEngine.Object>(assetPath);
					var existingLabels = AssetDatabase.GetLabels(asset);

					// Load the Preset
					var preset = AssetDatabase.LoadAssetAtPath<Preset>(presetPath);
					var presetLabels = AssetDatabase.GetLabels(preset);
					
					// Set labels
					var labels = presetLabels.Concat(existingLabels).Distinct().ToArray();
					AssetDatabase.SetLabels(asset, labels);
					AssetDatabase.Refresh();
					return;
				}
			}

			// Try again in the parent folder.
			path = Path.GetDirectoryName(path);
		}
	}
}