I know the question sounds weird, but is there any way to add sway using a plugin? I found that the cl_wpn_sway_scale and cl_wpn_sway_interp commands should be responsible for sway, but for some reason this ConVar doesn't mean anything in the game. And I found the source-files in which this ConVar has a variable and what and how it is responsible for. Predicted_viewmodel.cpp and Predicted_viewmodel.h. These commands are preserved in the game, as they are in client.dll (when compiling the dll, all the cpp and h files were used) in the bin folder.
https://mega.nz/folder/US4QGLjb#vnGU.../file/hLY1XA7K - there's the source (I think)