Just a few things I thought I would say...
1) Anyone noticed that depending on the complexity of the scene. Forest Pack objects can vanish, when doing a normal render, not just using IPR.
2) When using IPR, do you realise that al those point cloud FP objects, represent the full blown meshes, which are then only fully realised at render time. This explains, at least to me, why they would be vanishing, on hitting render and more so when using IPR, it's basically a memory thing. As iToo mentions... In the viewport, you have a certain number of items being displayed and in most cases, this is nowhere near the amount, that will be rendered, so that would mean, holding two versions of the Forest Objects in memory. The viewport version and the one being rendered. My guess is you would need a huge amount of memory, far above what the majority of average users have, in order to hold both, without causing problems.
3) As I understand it and correct me if I am wrong, Forest Pack was designed to help users scatter millions of objects around their scenes, but by representing them as point clouds / primitives, viewport interactions is kept at a usable speed. The ability however to render those objects in FULL, does mean you need to have a decent amount of memory on board. Al this stuff about being able to generate millions of trees, covering the ground in grass and wild flowers, rocks, etc.. and being able to render those scenes, is not strictly true. In the end it all depends on how complex your scene is and the amount of resources you have to render the scene, once it is ready.... someone with 16GB may have Max crash on a fairly simple scene, due to the face that while they are working on it, it may well handle the countless FP objects, but when it finally comes to rendering, they may not have enough memory to do so. Even with the 32Gb I have myself, I have had some scenes, which just would not render and have had to rework them in order to save memory.
Now add the ability, to not just render millions of items, but to also be able to interact with them, while you are rendering the scene, perhaps you will understand what it is you are asking and why, while it sounds like a very useful feature, I have a feeling it would just cause more problems than it would solve.
Sure if iToo can find a way to make it possible, I would be all for it. Only today I wanted to do a custom edit for a FP scatter, while using IPR, but as the popup dialog, it is not (YET) possible
I am not saying it cannot be made possible, I just think that it's going to be a tricky one and I don't know why, but as I said earlier, I see it causing more problems than it solves.. This is just me thinking out loud and I could be totally wrong. We can only wait and see.