Once again thank you for your great idea!
Yes,i t is to make exercises for students in an html page, so an image with empty Tab and to have the solution a click on button replaces the image with the solution and Tab filled.
So the images must have exactly the same dimension and bar of measures and spacing between staves must be identical .
The only small shadow to your idea is that when you hide the playing technique, the distance between the staves changes and we have to correct them each time vertically in the engrave page. Hiding the playing technique with the option not to recalculate the spaces would be perfect.
One solution is to select the playing technique and set an alpha to 0, but the disadvantage is that you have to save the slices in color and not in black and white, which gives quickly large images in an html page which contains a lot of them.
But in the end I kept your solution