I like the personable-ness (?) of "So." I mean, the tale of beowulf would have been told by bards around fires, or in loud mead-halls, or in dark taverns, not in formal settings. "So" is such a common-person way of getting a group's attention, or of starting a story with the rapt attention of all ears. It's not perfect, certainly, but I don't think it needs to be made bigger, more meaningful than what it is, a call for attention.
Well, there's more to it in my opinion. You're right about the general informality -- but since like you say Tolkien's works were commonly very biblical and grand we know what his preferences are. Also, 'Hwaet' has a strong consonantal sound which 'so' completely doesn't. There needs to be an abrupt harsh sound to grab the audience. My money's on behold. One thing that's refreshing is we know Tolkien's version has absolutely nothing to do with public opinion or the most recent translation, because it was written so long ago.
I'm surprisingly stoked about the whole thing. It will be interesting to compare his translation with heaney's and see what is different. What about "Well", or "Hey"?