In most cases, the best you can do with your buttons is to copy and paste them from layout to layout.
I suppose you might be able to set up a portal to a related record. Then adding a button to the portal would appear in all layouts where the same portal is set up...
I would like you to elaborate on that. I have layout objects, that are repeated in all layouts, but the number of layouts makes it time consuming to just copy and paste. So i would like to hear more about that portal methodd. And, preferably, that if i click it perfom a script, not just go to related record, when i click on that button, that should appear on all layouts.
What I've used (since version 4 I think) is a small rectangle object in the top left corner that I include when replicating objects across multiple layouts. I copy all of the objects (including the rectangle) I want to have appear in the same location on another layout. When you paste the items on the new layout you drag the group (which includes the rectangle) to the top left. The rectangle aligns the elements to the layout. It still requires that you adjust the individual parts of the layout to the same dimensions as your source layout but it works pretty well.