No, I don't think there is any way to display a custom dialog before the InstallShield prerequisites are installed. The prerequisite UI is controlled by the Setup.exe bootstrap, and most aspects of it are not customizable. (There are ways to tweak the strings on the prerequisite dialogs, but there's no way to add additional dialogs or perform tasks such as resize the dialogs.)
Er, well, perhaps you could create some special custom "prerequisite" whose sole purpose is to show the special UI, and schedule that prerequisite before your other prerequisites. I'm not sure whether this would fit your requirements, or how easy it would be to implement this.
Another option you may to consider is to switch to a Suite project. You would add each prerequisite package to your Suite project as a dependency package, and set up all of the conditions that help the new Suite Setup.exe bootstrap determine whether each package needs to be launched. With this approach, you have full control over the bootstrap UI, so you can incorporate your own custom UI wizard pages before things are installed.
