So, upon searching for new items for my fish tank, an idea dawned upon me....
Bear with me however, before you start to think I'm cheap (though I do admit, I must live on a budget) consider this:

So i was searching for a new cave rock structure for my fish when I thought, Why not use these in bonsai?

Looking through some rocks and structures I found a stonehenge looking cave thing #1
as well as a piece of beautifully shaped simulated driftwood #2.

Then I got to wandering these and found some more...

I guess what I'm saying is, has anyone ever used aquarium decorations or know of whether or not it would be safe to use with a growing tree

If it is safe for fish then it will be safe for plants. If it looks suiyable then fine, I have bogwood which was originally in a tank which I now have accent plants on. I am not sure which is your stonehenge looking cave, stonehange is nothing like any of those, but yes it looks ok. Real driftwood is always going to look better than fake, though.

Oh, oops! Just noticed that.. I had uploaded the wrong link... my bad!