We do not particularly allow script tags in our code as a security thing. Now we actually have a function called convert_html which does not strip bad tags. I made a change in adm-misc.php so it can use that instead of un_htmlspecialchars so you can redownload the build.
Anyway your talking about apples to oranges. Your talking the vb
banner ad manager not plugin manager.
Our plugin manager allows you to create plugins html banner ads in the format we noted.
Here is a tutorial on exactly how to create a plugin. New Plugin System FAQ!!!
Based on the tutorial adding for example a banner ad in the header bottom you would do something like this
Content visible to verified customers only.
I offer no support on plugins as we only support our code this is the very reason I posted such a long tutorial
If you read the tutorial you should be able to make all the plugins you want.