Your only real option is to find the offer you are running at a network that has support for iframe pixels on the campaign. You will not find a workaround that will allow you to call multiple pixels from a regular URL pixel.
You can't simply have a server side script that makes all the pixel calls, because when your server makes these pixel calls, it won't have access to the cookies that are needed by the pixel calls. The lack of cookie access will cause the multiple pixels calls to fail.