I know feasibility studies are recommended by a number of project management methodologies i.e. The Waterfall Model, but would a feasibility study be method for project management?
No and No
Have a look at the definition of a feasibility study (e.g. in Wikipedia): ‘A feasibility study is an evaluation of a proposal designed to determine the difficulty in carrying out a designated task.’ Hence a feasibility study can be part of a project. Feasibility studies are not part of a methodology. But some methodologies give advice on how to handle projects which have feasibility studies as they introduce a higher level of uncertainty and uncertainty makes planning more difficult.