I am about to do this same thing today... and was asking myself the same question... suspecting the answer you gave.
So, here was my thought, make the lactose concoction and put it in a second corny keg, then use a jumper tube to force it into the stout keg. I'll lower the pressure on the stout keg, then as long as the pressure is higher in the lactose keg it should transfer fine, no muss no fuss!
I'll let you know how this all works out.
What is 'muss' anyways?