Is it possible to always direct users to the Welcome Page in SharePoint, when using login as an other user or when requesting a sub site, page in the browser?
In that user control, you could set a cookie or session variable that says "I've already redirected this person" so that each time your code runs, it first checks to see if this has been set and if not, redirect the user after setting it.
I can advise you to craete custom content page with custom code behind logic and after deploy set this page a Welcome page.
This way you will have a full control about what you are doing and how users are redirected.
Hope it helps,