How to blend images on a spherical surface / cube map
I have eventually found a simple idea for the solution on the paper called "Blending Images for Texturing 3D Models".

The idea is not doing the complete blending for each new image added to canvas, but rather build separate lowpass and highpass canvas (for two-band blending). These are then merged as a final step of the rendering.

Finding seam is still a bit tricky and not used in the paper. It requires reprojecting data from canvas to image, computing the seam and then projecting the blending mask to the canvas.

The method described in the paper is simple and does not leave any blending artifacts.

The only drawback is that more data have to be kept in memory.



Update

This solution does not work if the rendered canvas is smaller than original image. Furthermore gradient-domain blending is not easily applicable as well.