Alrighty, I've re-read the story and feel I can offer a few comments now

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First, I'm not entirely certain that Beren needs to be entirely speechless until he spies Luthien. The lack of extensive dialogue in the story is due more to Tolkien's storytelling style than any particular character trait. Having said that, there wouldn't be much of a need for dialogue between the time Beren discovers his father's death and later comes across Luthien.
I agree, he should seem brooding and desperate particularly during this period, but with a noble and yet humble soul, in short, much like Viggo's Aragorn. I don't really know how old he's supposed to be, but I like David Thewlis for the part (he plays Remus Lupin in the Potter films), but at 46 he may be too "old". Shame that.
I love the idea of Beren having premonition dreams about Luthien during his Orc killing spree Odo.
Now Huan should be a large dog, big enough for Luthien to ride on. Possibly an Irish or Russian Wolf-hound with special effects to make him look bigger. I'll check my dog books and when I have a firmer idea I'll try to find some pictures to post.
Interestingly, Huan is limited to speak only 3 times in his life. Not certain the best way to handle that yet. I actually liked the way it was done in the Narnia films, but would it work in a film geared more for adults? Perhaps, we are talking about making the film for fans of the story, so they would be prepared for that.
As for the singing battles, that could present a problem. I like the music from many musicals, and have no problem with it when they are stage productions. But it takes me right out of a story when people sing their dialogue, so I'm not a big fan of musicals as realistic drama. But i am thinking that if it is presented as wordless vocal Magic--maybe it could be pulled off.
That is to say, the characters would just vocalize certain tones in an operatic manner, perhaps electronically enhanced and we could see the sound waves rippling through the air and vibrating the surrounding props and set until the resonant frequencies shatter certain objects.
Anyway, just a few thoughts from me to start.
GB