Okay, after fiddling with the examples it turns out the rendering is fine. I suspect what is happening is that it's being drawn outside of the bounds of the texture, hence why I'm not seeing anything.

That being said, there's some unexpected behaviour when the skeleton is a child of a node. In the spineboy example I set the skeleton to be a child of a node and I set the position of this node to the original position. I then set the position of the skeleton to zero. When the render to texture is complete, the skeleton jumps to a world position of 0,0 and the rendered texture positions the skeleton at 0,0 in the image. Below is the modified code:

//SpineboyExample.cpp
//...
skeletonNode->setTrackStartListener(jumpEntry, [] (spTrackEntry* entry) {
      log("jumped!");
   });

   Node* node = Node::create();
    node->setPosition(Vec2(_contentSize.width / 2, 20));
    addChild(node);
    

   //skeletonNode->setPosition(Vec2(_contentSize.width / 2, 20));
   node->addChild(skeletonNode);

   scheduleUpdate();

EventListenerTouchOneByOne* listener = EventListenerTouchOneByOne::create();
   listener->onTouchBegan = [this] (Touch* touch, Event* event) -> bool {
        const float reducedTimeScale = 0.3f;
      if (!skeletonNode->getDebugBonesEnabled())
         skeletonNode->setDebugBonesEnabled(true);
      else if (skeletonNode->getTimeScale() == 1)
         skeletonNode->setTimeScale(reducedTimeScale);
        else if(skeletonNode->getTimeScale() == reducedTimeScale)
        {   

            auto testRenderTexture = RenderTexture::create(1024, 1024);
            testRenderTexture->begin();
            skeletonNode->visit();
            testRenderTexture->end();
            testRenderTexture->saveToFile("test.png");
        }
      else
         Director::getInstance()->replaceScene(GoblinsExample::scene());
      return true;
   };

Any ideas/best practices on correctly placing the skeleton on a texture and also creating the acquiring the correct size for the texture (other than trial and error)?

So I've managed to get it to render to the texture... almost completely correctly... at the very least it's in the right position.

However, it would appear that not all of the skeleton is rendered as you can see below (there are supposed to be arms and legs):

Image removed due to the lack of support for HTTPS. | Show Anyway

The artist who made this has indicated that the limbs are:

... meshes with weight bindings on the verts. Everything else is a sprite..

Any ideas on how to correct this?

EDIT
I've managed to reproduce the error with the tank example. Simply apply similar code from the snippet I provided above. Everything except the aerial renders correctly.

Image removed due to the lack of support for HTTPS. | Show Anyway

Workaround
What we've done to get around this is to create a one frame animation, hide everything except optional attachments and drop in a sprite of the whole skeleton. This is fine so long as you don't want a "freeze action" shot, more of a pose.

Using the tank example, rather than the spineboy example

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#include "TankExample.h"
#include "BatchingExample.h"

USING_NS_CC;
using namespace spine;

Scene* TankExample::scene () {
   Scene *scene = Scene::create();
   scene->addChild(TankExample::create());
   return scene;
}

bool TankExample::init () {
   if (!LayerColor::initWithColor(Color4B(128, 128, 128, 255))) return false;

   skeletonNode = SkeletonAnimation::createWithJsonFile("tank.json", "tank.atlas", 0.5f);
   skeletonNode->setAnimation(0, "drive", true);

   skeletonNode->setPosition(Vec2(_contentSize.width / 2 + 400, 20));
   addChild(skeletonNode);

   scheduleUpdate();
   

   EventListenerTouchOneByOne* listener = EventListenerTouchOneByOne::create();
   listener->onTouchBegan = [this] (Touch* touch, Event* event) -> bool {
      if (!skeletonNode->getDebugBonesEnabled())
         skeletonNode->setDebugBonesEnabled(true);
      else if (skeletonNode->getTimeScale() == 1)
         skeletonNode->setTimeScale(0.3f);
        else
        {
            auto testRenderTexture = RenderTexture::create(1024, 1024);
            testRenderTexture->begin();
            skeletonNode->visit();
            testRenderTexture->end();
            testRenderTexture->saveToFile("test.png");
            Director::getInstance()->replaceScene(BatchingExample::scene());
        }
      return true;
   };
   _eventDispatcher->addEventListenerWithSceneGraphPriority(listener, this);

   return true;
}

Yeah, I'm inclined to agree as I saw some funny UV stuff going on as well and I observed no difference in the logic when stepping through it.

Question now is do we (the users) or you provide a workaround, or contact the Cocos guys and tell them something is off with their off screen rendering?