Xilinx recently created (then revoked) an open-source tool called "RapidWright" that might give you an idea -- it was basically a big Java API for manipulating and controlling Xilinx DCPs/bitstreams, allowing for some interesting tools. They plan on re-releasing it[1], but you can still see the docs here:

  http://www.rapidwright.io

The most interesting part are the tutorials, where they automatically e.g. insert ChipScope ILAs into existing, routed designs, which can only be done with some knowledge over the routed/placed results. (The automated UltraScale+ SLR-crossing example is probably more _interesting_ from a technical POV, but admittedly the ChipScope example was something I wanted recently!) Granted, that's not exposing the level of detail you'll need to write a full place-and-router or anything, but post-route/route-assistance based tooling seems like an interesting possibility.

Honestly, though, I'll take absolutely none of that if an open source tool means you can just fix some of the bugs in the damn things.

[1] I assume they revoked it in the first place due to some outbreak of batshit insanity in a legal department somewhere -- presumably some of the source code should not have been released, or someone's mind changed, or something.