Old Double Hung Window Weight Pocket Inaccessible From Inside

Hi Guys, Just getting started on this 1923 renovation and as I pulled the windows apart to start restoring, I notice that the plaster covers the weight pockets on either end of the multiple window bay. Now, I suppose I could get into there by prying off the trim on the outside, but I'd much rather have these be accessible from the inside in the future. Is there any problem anyone can foresee to me just cutting a hole in the plaster (which is covered by the side casing) so I can get to the weights from the inside? Photo attached. Image: https://farm4.staticflickr.com/3953/15508403026_4af7351766_c.jpg