any time consuming bandwidth eating enhancement you make, will make no difference to the user,
If the enhancement delays the loading of the page they will leave
user click away may seriously annoy the client who is paying for this development
Check with the client whether they want a inaccessible site
before you code, much flash is not disability friendly, client location may limit what can be done, or force parallel coding
it has to be simple to maintain as well
If you pitch it at them right, and the idea is different enough, and effective, you'll get the go ahead, and there won't be questions later about how much extra were these features we didnt ask for.
the sort of script you need is likely already available to plugin from http://www.flashcomponents.net I have used this one. there is no noticeable page delay over the static jpg image that used to be there.
There are a bunch of user contributed actionscripts, on forums like this, but I cant at the moment think of what the google search would be