I now have my XSLT module successfully reconstituting a line-broken word on both sides of the break, like this:

<ab corresp="mar:textnode#xpath(/*[1]/*[2]/*[2]/*[1]/*[2]/text()[10])"><seg>
                    </seg><w corresp="mar:offset#xpath(substring(., 22, 3))"><choice><orig>ant</orig><reg type="joined-2">imagiant</reg></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 26, 3))"><choice><orig>que</orig></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 30, 4))"><choice><orig>Vous</orig></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 35, 5))"><choice><orig>pren-</orig><reg type="joined-1">prendrez</reg></choice></w></ab><ab corresp="mar:textnode#xpath(/*[1]/*[2]/*[2]/*[1]/*[2]/text()[11])"><seg>
                    </seg><w corresp="mar:offset#xpath(substring(., 22, 4))"><choice><orig>drez</orig><reg type="joined-2">prendrez</reg></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 27, 7))"><choice><orig>quelque</orig></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 35, 8))"><choice><orig>intereſt</orig></choice></w><seg> </seg><w corresp="mar:offset#xpath(substring(., 44, 1))"><choice><orig>à</orig></choice></w></ab>

It's nasty-ugly but it's only intended for machines to read. Having the full form of the word on both sides of the linebreak means we'll be able to do n-grams properly, and having the two joined forms labelled differently (joined-1 and joined-2) means we'll be able to ignore one of them if we're reconstituting a continuous string.