NVIDIA and Epic Games have just announced that Unreal Engine 4 developers can view and modify the source of PhysX. This also includes the source for APEX, which is NVIDIA's cloth and destruction library. It does not include any of the other libraries that are under the GameWorks banner, but Unreal Engine 4 does not use them anyway.
This might even mean that good developers can write their own support for third-party platforms, like OpenCL. That would probably be a painful process, but it should be possible now. Of course, that support would only extend to their personal title, and anyone who they share their branch with.
If you are having trouble finding it, you will need to switch to a branch that has been updated to PhysX 3.3.3 with source, which is currently just “Master”. “Promoted” and earlier seem to be back at PhysX 3.3.2, which is still binary-only. It will probably take a few months to trickle down to an official release. If you are still unable to find it, even though you are on the “Master” branch, the path to NVIDIA's source code is: “Unreal Engine/Engine/Source/ThirdParty/PhysX/”. From there you can check out the various subdirectories for PhysX and APEX.
NVIDIA will be monitoring pull requests sent to that area of Unreal Engine. Enhancements might make it back upstream to PhysX proper, which would then be included in future versions of Unreal Engine and anywhere else that PhysX is used.
In other news, Unreal Engine 4 is now free of its subscription. The only time Epic will ask for money is when you ship a game and royalties are due. This is currently 5% of gross revenue, with the first $3000 (per product, per calendar quarter) exempt. This means that you can make legitimately free (no price, no ads, no subscription, no microtransactions, no Skylander figurines, etc.) game in UE4 for free now!