How to add tinymce parameters to plugin?

I have figured out that in order to enable my editors to paste iframe code into a page they either have to use a chunk or other non-formatted method (which is not ideal) or I have to add an extended_valid_elements parameter to the tinymce init.

eg.,

[tt]tinyMCE.init({
...
extended_valid_elements : "iframe[src|style|width|height|scrolling|marginwidth|marginheight|frameborder]"
});
[/tt]

(http://wiki.moxiecode.com/index.php/TinyMCE:Configuration/extended_valid_elements)

Can I customize my modx plugin with this type of parameter? What’s the syntax? When I try to put it in custom parameters it gets stripped out.

Thanks