Will it wreck the dish if I pre-soak beef in brine, and then slow roast with Marsala?
Do it! Marsala is sweet, brine is salty. You may be on to something here...

Really though, I always brine anything I roast or sous-vide. No issues here. 

EDIT: I would recommend using olive oil instead of butter if you are still worried. Not to mention Marsala + olive is delicious.