Theres a trick which a makes it very easy (and ist been there since forever). In fact the solutuion is even better then your wish, since you get to see over the seam getting you even better visual feedback.
What you do is you use filter -> other -> offset and use this to move the seam to middle of yoru canvas. Then do that area , offset again for the veruu small area in corner and then offset it vback to where you started from.
But yes a plugin that would make the canvas 9 times larger and then repeat the subregion 9 times could be done. Its jst that its unnessereily much work for a very slight improvement (you wouldnt pay enough). Youd actually trather want to use a autiomatic seam killer, which is easy to amke just fourier ttransform the data do abit of filtering and your done.