Clive Watkins had a series of threads on Thomas a couple years ago, including Clive's own very interesting commentary on some of the poems:
http://www.ablemuse.com/erato/showthread.php?t=748
http://www.ablemuse.com/erato/showthread.php?t=750
http://www.ablemuse.com/erato/showthread.php?t=753
Regarding "The Unknown"…
Like Petra, I read the poem as falling into a 2-3-2-3-2 stress pattern, but the consistency of the syllable counts is notable, 4-6-5-6-4, the one exception being the line Philip points out (if an elision is allowed in line 14). In particular, the extra syllable in the middle dimeter line seems to have a deliberate rhythmic effect, giving a feminine ending to the one line that doesn't rhyme. Very "crafty."
As to who "she" is, maybe the title is the best answer. She is that which, grasped, would lose all value.