Perhaps I’ve missed an obvious existing solution?
As I understand it, the equivalent of Liquify is the UV Brush SOP—deforming the UV map and sampling it. The tool is needed in COPs but resides in SOPs. You can't view the COP texture and the SOP tool simultaneously, so you have to create a dense grid, generate UVs, warp them with the UV Brush, and send the output along. Then, you need a parallel grid branch (to avoid recursion) using a COP Preview Material that references the sampled, deformed texture from the COP. You set the render flag to the COP Preview Material while keeping the UV Brush active. Now you can see the texture and deform it, but the edges get in the way; you have to find the option to disable "current geometry" in the right-hand panel menu (which isn't very intuitive), or—easier yet—simply offset the COP Preview Material slightly so you don't have to keep toggling "current geometry" in the menu.
This works well, provided the texture is square. If it’s rectangular, you have to adjust the grid's proportions beforehand. Hello, Python.
So, technically, there are all the tools to replace Liquify, but collecting and using them is a rather complicated task.
I hope the developers make using this tool within COPs simpler.
