let $x := unparsed-text-lines(...)

return ($x[1000], $x[10])

Saxon implements $x using a data structure called a MemoClosure where items
from the input are read on-demand, and then remembered. Reading $x[1000]
will cause the first 1000 items to be read and retained in memory; reading
$x[10] finds that the tenth item is already in memory

Maybe not store the 1000 lines, except the last, but just create a
map N --> offset(N) for the Nth line, N < 1000