> Off the top of my head I'm not sure we have (or could) encounter
> such a case in prose. (Perhaps items of a list or some such?)

Gotta love XPath. I just took a quick perusal through the WWP
textbase and found that of the <lb> elements that indicate the line
is continued above or below baseline:

           count    parent of <lb> that's above or below
           -----    ------ -- ---- ------ ----- -- -----
            1846    l
              43    item
              17    said
              12    p
              11    stage
               7    placeName
               6    persName
               3    emph
               2    rs
               2    mcr
               2    add
               1    unclear
               1    titlePart
               1    supplied
               1    roleDesc
               1    quote
               1    name
               1    head
               1    docImprint

So while the vast majority are in poetry, there is definitely a prose
contingent. The 69 <item>, <p>, <stage>, <titlePart>, <quote>, and
<docImprint> are not verse -- the others might be. Thus:
   94.2% -> known verse
    3.5% -> known prose
    2.2% -> undetermined by this simple test