Like you, I made regular use of the premain insertion area in weaver pro, very useful for all sorts of things – especially header images and slideshows. I quickly found that xtreme plus has two areas that easily replace premain.
Either postheader or container_top should meet your needs, they certainly work well for me. Here is a site I am currently building with xtreme plus, making use of postheader to add a different header section to most posts/pages. http://dev.creativesurge.co.uk/image/