Taking a screenshot from a secondary off-screen camera?

I have been trying create a system for an iOS game that will generate a random photo that users can then share over social media via the Prime31 Social Networking plugin. I am creating the image that I would like for users to be able to share using various Gameobjects. They are all being rendered off-screen but while the game is running.

It seems like I should be able to do this very simply, however I am struggling to find a way to generate a PNG out of my off-screen content. I used the little documentation available on this topic to piece together the attached script. As far as I have seen, everyone is focused on taking screenshots of content that is in view of the main camera and therefore also to the user.

My script currently crashes Unity, I know this is caused by the invalid negative parameter for the new rectangle I am trying to create off-screen, but I wanted to illustrate what I am trying to do.

So to sum up my questions include, is it even possible to convert my off-screen content to a PNG? How would I go about generating a PNG from the content if it is possible? Is my current method the best way to capture that content and package it for use with the Prime31 plug-in?

Other notes, I don't have Unity Pro so the RenderTexture method of taking screenshots is out of question, even though I'm not sure if it could be used to solve my problem.

Attached code:

 using UnityEngine;
 using System.Collections;
 using System.IO;
 
 public class ScreenShot_Taker : MonoBehaviour {
     
     Camera screenshotCam;
     RenderTexture renderTex;
     int width = 200, height = 200;
 
     void Awake() {
         screenshotCam = GetComponent<Camera>();
         renderTex = new RenderTexture(width, height, 24);
         screenshotCam.targetTexture = renderTex;
     }
 
     Texture2D CaptureScreenshot () {
         RenderTexture tempRT = RenderTexture.active;
         RenderTexture.active = renderTex;
         screenshotCam.Render();
         Texture2D screenshot =  new Texture2D(width, height, TextureFormat.RGB24, false);
         screenshot.ReadPixels(new Rect(-38.85f, 5f, width, height), 0, 0);
         screenshot.Apply(false);
         RenderTexture.active = tempRT;
         return screenshot;
     }
 
     void SaveScreenshot() {
         string filePath = Application.dataPath + "/screenshot.png";
         File.WriteAllBytes(filePath, CaptureScreenshot().EncodeToPNG());
     }
 }