While the means that have been described will undoubtedly work in some manner, it seems to me that the desired effect is more easily obtained in the darkroom with the use of masking of the camera negative.
The reason that I make that statement is that the matter of cutting a mask to place over a camera lens is an onerous and imprecise method because of the matter of scale. With masking of the camera negative in the darkroom the effects of scale are greatly reduced and even eliminated if wished. If you have not already done so, I would suggest the articles written by Howard Bond in PT magazine on "Burn and Dodge" masking. This procedure is one that he learned from Alan Ross who does a lot of printing of the Ansel Adams photographs.
While there is no one to tell you that you can't or even shouldn't pursue your methodology, it just seems unproductive to reinvent the wheel.