[jQuery] Centering simple-modal in an iFrame

hi,

we have a page A with an embedded iframe B (which is quite long, but
most of the time the content only fills the part at the very top).
Displaying a simple-modal centered in the iframe B results (obviously)
in the modal being located in the non visible area. So we're wondering
whether there is an alternative to that, e.g. centering the modal in
the visible part of the iframe.

Thanks for your help, any thoughts are appreciated!