>I see the parallel imagery with "Preludes," but Eliot used and reused
>and reused images throughout his work.  They are not just repetitions
>but re-creations, I think.

Yes, that is why I called them "reworkings" rather than "repetitions."

>I also do not see how this brings us any closer to a reason for reading
>the poem as symbolic of the struggle to write.

It probably doesn't.  However, I think that spending "hour on hour of 
prayer" and feeling one's soul "stretched tight across the skies" are 
experiences of struggling with something.

>I think it is about what it depicts.

I've just come across an article by Richard A. Kaye in 
_Modernism/modernity_ (Vol.6, No. 2, April 1999, p. 107-134) that 
says: "the St. Sebastian poem afforded Eliot the opportunity to 
assert a resolute modernist stance in opposition to a leading strain 
in late-romantic poetics" (112).  Carrol, you might find it 
interesting that on the same page, Kaye says:

       if, as John Berryman claimed, the title of 'The Love Song
       of J. Alfred Prufrock' is 'intensely anti-romantic . . . a rebuke
       to the (probably romantic) reader,' that rebuke is even
       greater for the implied romantic reader of 'The Love Song
       of St. Sebastian.'  [Ellipsis is Kaye's]

Kaye's citation to Berryman reads: John Berryman, "Prufrock's 
Dilemma," in _The Freedom of the Poet_ (New York: Farrar, Straus, and 
Giroux, 1972), 273.

> In any case, Eliot talks about the poem in a letter to Conrad Aiken (25
>July 1914) for what its worth--pp 44 and 46-7 in the Letters.  He
>suggests that the poem is about the feelings of a St. Sebastian and
>raises the question of homosexuality in order to deny it.

As it happens, the title of Kaye's article is "'A Splendid Readiness 
for Death': T. S. Eliot, the Homosexual Cult of St. Sebastian, and 
World War I."  On page 118 Kaye says that Eliot "effectively 
transformed the martyr from a figure of specifically homosexual 
significance into one suggestive of a more generalized perversity 
that was nonetheless recognizably nonhomosexual."