Interesting. I'm not sure if you can do it with a compression fitting but my plumber did it this way.
We had no access either from underneath. He measured, measured and measured the entire bath drain assembly. Soldered it on and then lowered the tub in. He did it and I was amazed. I was able to lift the plywood from under the floor but it was about 3 feet away. We filled the tub and watched it drain from the panel. I looked at the connections and didn't see any water leaking.
Hope this helps.