When I try to do this on the master page, whether I use RegisterStartupScript or Response.Write, whether I put it in Page_Load, Page_PreInit of the master page, the script doesn't show up in the page source at all.
I want this flash object to appear at the top of every page in the site, so I thought it made sense to put it in the master page. Is there a way to accomplish this?
*edit* - On further investigation, I've found that I can get this script early enough on the page to declare and assign the "clientID" variable before my script that uses it, but now my problem is that my script that reads the hidden field is executing before the hidden control itself gets rendered. So I no longer have a problem getting the clientID of the hidden field, I just don't have the hidden field in time to read it's value. My quest continues...