Tight grain in tandem with hardness should absolutely count, I think, if you want good tone from naked (that is, unvarnished or unlacquered) wood. There are flutes around made from sugar maple - a significantly hard and close-grained wood - and fruitwoods such as apple or pear have been used traditionally; but I have tried none of these, myself, so can’t report my subjective impressions of them. Boxwood is still a time-tested choice obviously worthy of favor as evidenced by the many who love it. I had a keyless Olwell made of dogwood at my request, and thus a prototype as Pat counted it, that played like a champ. All these woods have the characteristic of a fine and sometimes almost indistinct grain, and relative hardness. Not being an expert, my gut tells me you’d need both hardness and tight grain for a successful flute wood, not just either one or the other. While oak is hard, its grain is not at all finely dense, but coarse; I imagine playing an oak flute would be not unlike playing a carpet, to put it in hyperbole.
Terry’s experiment with Pinus radiata gets us close to the extreme other end of the spectrum, as its wood is both soft and of coarse grain. Have you tried balsa yet, Terry? 
Sidetracking momentarily from the issues of tone alone, I think there is an entirely practical reason accounting for the proliferation of the commonly used Dalbergia species since I think there’s reasonably good argument that those species are not necessarily inherently tonally “superior” to other astute choices of material, and that reason for their being standard flutewoods is instead in their oily and resinous qualities: nonresinous woods absorb and desorb moisture far more rapidly than integrally oily, resinous ones. My former dogwood flute is an exact case in point. While lighter in weight than blackwood, like any good and dense wood not only did it offer a great tone, it was hard as nails (it hit a tiled floor squarely on the edge of its endcap after rolling off of a standard-height table and didn’t show so much as a mark!), and the grain was so fine you could hardly even see evidence of it, but, being a “dry” wood, after a couple or so hours of playing, the barrel tenon in particular would swell so much (depending on the weather and the degree of wrapping) that disassembly sometimes threatened to be an exercise in power that was best left untried. Fortunately its desorption time only took about three or so hours if I felt it was better to just let things be. I was always having to try to plan ahead as regards the tenon wrapping, and I often set the pre-played tenon/socket fit looser than I liked in order to account for swelling, but it was a workable state of affairs as I could count on the fit getting nicely snug in short order. Having played blackwood flutes with much slower absorption characteristics previous to that, this was a novel experience for me, and it drove home an undeniably practical reason, based really only just on convenience and having nothing to do with tone, for going with the comparatively stable response to moisture that comes with resinous exotic hardwoods. So, we need to define “superior”. After all, tone in the case of dogwood wasn’t even an issue.
By the way, Pat O. had pressure-treated that dogwood flute for a week in linseed oil before I got it, and I continued thereafter to oil it liberally and most especially at the exposed grain ends of the tenons as well as the embouchure cut, too - you could watch the oil being thirstily drunk in in no time at all every time - but this frequent oiling didn’t seem to significantly affect moisture absorption or desorption all the same. Conversely, I’ve noticed that my present blackwood flute absorbs no oil perceptibly at all that I can tell, so I rarely even bother any more and just mind its state of humidification. I’ll probably give the tenon ends a good oil soak this year before winter sets in, but I think this will be more to satisfy me than the flute.
Of course the go-to classic good looks of exotic hardwoods, and the convenience of their moisture resistance and ability to shed condensation well, is balanced by the toxic nature of the oils and resins themselves with their concomitant potential for player contact allergy, to say nothing of the risks makers face in working such woods.
But, back to the Flute Tone Investigations, and sorry for the weighty sidetrack. 
