When and how to take a screenshot of a particle system from the editor?

I am writing an editor extension that, among other things, traverses the hierarchy of an object and takes thumbnail images of the components, depending on type. If it finds a texture, it writes the texture to a small PNG. If it finds a material, it applies the material to a model, instantiates it, takes a screenshot, and writes that screenshot to a small PNG. So far so good.

My problem is getting a snapshot of the particle system. I want to get to a time value of 0.1 seconds in the particle system and get a shot of what that looks like. Here's my code so far, with some irrelevant parts snipped.

     DirectoryInfo ThumbBundlePack(Object asset) {
         //...snip

         //Explicit handling for types
         switch (asset.GetType().ToString()) {
             case "UnityEngine.Texture2D":
                 subdir = MakeSubdir();
                 //..snip
                 WriteThumbnail(asset as Texture2D, subdir);
                 MakePackage(asset, subdir);
                 break;
             case "UnityEngine.Material":
                 subdir = MakeSubdir();
                 json.AddTags("material");
                 WriteThumbnail(asset as Material, subdir);
                 MakeBundle(asset, subdir);
                 MakePackage(asset, subdir);
                 break;
             case "UnityEngine.Shader":
                 subdir = MakeSubdir();
                 json.AddTags("shader");
                 MakePackage(asset, subdir);
                 break;
             default:
                 if (listed) {
                     subdir = MakeSubdir();
                     WriteThumbnail(asset, subdir);
                     MakeBundle(asset, subdir);
                     MakePackage(asset, subdir);
                 } else {
                     Debug.Log("Didn't handle: " + asset.ToString());
                 }
                 break;
         }

         return subdir;
     }

 void Snapshot(DirectoryInfo dir) {
         //Has main camera save whatever is in the scene to the specified directory.
         
         //Set up camera to take picture
         RenderTexture shot = new RenderTexture(256,256,24);
         Camera.main.targetTexture = shot;
         Camera.main.Render();
         RenderTexture.active = shot;
         Texture2D tex = new Texture2D(256, 256, TextureFormat.RGB24, false);
         tex.ReadPixels(new Rect(0,0,256,256), 0, 0);
 
         //Write file
         using (BinaryWriter bw = new BinaryWriter(File.Open(dir.ToString() + "/thumb.png", FileMode.Create))) {
             bw.Write(tex.EncodeToPNG());
         }
     }

The version of WriteThumbnail() for textures - no problem:

     void WriteThumbnail(Texture2D tex, DirectoryInfo dir) {
         //Copy texture to readable so it can be manipulated
         string tmptx = "Assets/3damsTempTex.png";
         AssetDatabase.CopyAsset(AssetDatabase.GetAssetPath(tex), tmptx);
         TextureImporter ti = (TextureImporter) TextureImporter.GetAtPath(tmptx);
         ti.isReadable = true;
         ti.compressionQuality = 30;    //0-100
         ti.textureFormat = TextureImporterFormat.ARGB32;
         ti.mipmapEnabled = false;
         AssetDatabase.ImportAsset(tmptx, ImportAssetOptions.ForceSynchronousImport);
         Texture2D thumb = (Texture2D) AssetDatabase.LoadAssetAtPath(tmptx, typeof(Texture2D));
 
         //Scale
         TextureScale.Bilinear(thumb, 256, 256);
 
         //Write file
         using (BinaryWriter bw = new BinaryWriter(File.Open(dir.ToString() + "/thumb.png", FileMode.Create))) {
             bw.Write(thumb.EncodeToPNG());
         }
 
         //Clean up temp asset
         AssetDatabase.DeleteAsset(tmptx);
     }

The version for materials - no problem:

     void WriteThumbnail(Material mat, DirectoryInfo dir) {
         //Apply material to default model, put in scene
         GameObject model = (GameObject) Instantiate(AssetDatabase.LoadAssetAtPath("Assets/Editor/3dams/cube_bevelled.fbx", typeof(Object)));
         model.renderer.material = mat;
         
         Snapshot(dir);
         DestroyImmediate(model);
     }

Here's the trouble one - the version for generic objects, which checks to see if a particle system is attached:

 void WriteThumbnail(Object prefab, DirectoryInfo dir) {
         //Generic case, with special considerations for particle systems.
 
         GameObject gobj = (GameObject) Instantiate(prefab);
 
         if (gobj.GetComponent<ParticleSystem>()) {
             Debug.Log("Found particle system in generic object");
             ParticleSystem particles = gobj.GetComponent<ParticleSystem>();
             particles.Play();
             particles.time = 0.1f;
             particles.Pause();
         }
 
         Snapshot(dir);
         DestroyImmediate(gobj);
 
     }

This works fine for textures and materials. But the screenshots for particle systems are showing up blank, just the background color of the scene. Any idea why, and what to do about it?