Bob (Robert) Le'Chevalier (aka la lojbab.) is one of the developers who in 1987 constructed the famous Lojban language.
Bob is sometimes the last source of information regarding intents behind Lojban (together with the official Reference Grammar).
And so in this section I will be posting my questions to Bob, his responses and my comments.

La Gleki: - As Curtis [one of Lojbanists] noted the U.S. American "frown" primarily means a downturned mouth and usually means negative emotions. The British sense is just a furrowed brow (forehead), which can mean all sorts of things, including negative ones but also (but not limited too): a reaction to pain (possibly covered in the Usian sense too, as a grimace), concentration, a questioning look, a prompting look, or surprise. So what was meant by "frumu". Should we stick to "grimace" reading more, that is any facial expression would do?
la lojbab.: - I suspect that grimace would work, since it an alternative to frown in the definition. Until I looked them up (see below), I would have thought grimace to be a synonym of frown. But that seems not to be the case.
I don't know the answer to this for sure.
Since the question is one of various meanings in different languages/cultures, it is probably best to look at the source words that were used to make frumu. mukti scanned the original worksheets when he was last here. I don't know if he put them online yet. Actually I think we put the etymology summary file up a while back. But the original sheets will identify the words we used in word-making that did NOT make it into the final etymology.
Thinking about it, I probably had in mind "frown" as the opposite expression to "smile", which would fit what Curtis called the American version. But I think that the American definition also includes the furrowed brow as well as the downturned mouth, and could therefore include concentration and disapproval as well as sadness.
Hmm.
http://dictionary.cambridge.org/us/dictionary/english/frown
http://dictionary.cambridge.org/us/dictionary/english/grimace
allows you to look at the British and American definitions in comparison. There is some difference, but I think I would have frumu cover both the American and British meanings of both words.
Broader is better for gismu, especially if we can find a way to make simple lujvo to distinguish between kinds of frumu. drifru for a sad frown; na'efru for a denial/disapproval frown, jurfru and jusfru for the to-me subtle difference between a serious expression and a severe one (subtle in that I couldn't tell you the difference in the facial expressions for each). So that is what I would propose - that frumu be a kind of basket of non-smiling facial expressions, not necessarily for a negative emotion, with lujvo bringing out the various possible kinds of emotion/attitude being expressed.