Martti: there's a really pale powder called 'rice powder'. You might try that? For foundation, have you tried 'Siberia' from Nars? There's also Mac's 'Studio Fix' NW10.
Have you considered actual stage makeup? The sort clowns use for whiteface, not the greasepaint but the pressed powder stuff, which is applied with sponges. You can also dilute it with water to get different degrees of paleness. With an ordinary pale foundation, you can get a nice pale effect by dusting lightly with cornstarch before it dries. This takes a bit of practice to get right, so don't try it for the first time just before you go on stage! And hold your breath! Some people use talc the same way, but it's nastier to breathe.
