Hi tfs,
Here are a couple options:
1. make a 900x100 flash movie (.swf) that includes your header image and your existing flash movie that you want to overlay. You can use the %flash[url,width,height] in the Configure header options. See the article
'Add a flash header to Wordpress Atahualpa theme (3.4.4)'. It works good and no code to write, just replace bfa_header_config.php with supplied file. Also, there is a sample flash source file (.fla) that you can use as a starter.
2. Another option is to use the %html[.....]% param in the Configure header area options of the Atahualpa Theme options. See '
Add Custom Header to WordPress Atahualpa Theme (ver 3.4.4)'. Again, replace the bfa_header_config.php file with the supplied. By using the %html[...]% param, you can supply HTML for your flash movie and the image you want to display in header. Or you could use the %image param for your image and use the %html[...] param to handle a relatively positioned (by css) object element for the flash overlay. The sample code in the %html[...]% example uses a relatively positioned flash movie overlay similar to what you are looking for.
It really depends on your skill set and ambition. If you are reall good at flash, I might do #1 which is pretty clean. If you are good at HTML and CSS (inline) and want to use %image or want a true overlay, try option #2.
Good Luck with it!
note: The %flash[...] and %html[....]% params add quite a bit of flexibility to header and don't require .php hacks.