Ok, i created my own MyCms module and a block class: Mage_MyCms_Block_Breadcrumbs class (just as the breadcrumbs class in the catalog/blocks). Also I added this class to the local.xml config. This class overrides the _prepareLayout function, tries to get the breadcrumbsblock and modifies it.
Now comes my big problem. What to do now? How do i make sure that on a cms page the breadcrumbs are actually displayed. Im missing a very important step here. Its actually related to my not understanding on what basis the breadcrumbs block is loaded/added to the layout in the catelog module.
Well, im halfway building it in myself and would very much appreciate a more concrete answer to my question As far as i get it now, the controller uses the main.xml to create a Layout page and loads all the blocks that are specified in the layout. Is this correct? Moreover, it cant work exactly like this, cause i dont think i could include *any* block at any point in my main.xml. Ie. I dont think a particular checkout block would work on the frontpage. So what limits the use of particular blocks?
And if it works like that, then adding my custom breadcrumbs block to the cms layout part would mean that it will load that block and render it?