Is it possible to have the 8va line extend to the end of the piece?
Yes, you should be able to either lengthen an existing octave line, or select the whole stave where you want it and then input one spanning that selection.
Alternatively, use an octave-transposing clef and use the option for it to impact transposition.
Thank you, Lillie. The simplest, of course, is your second suggestion (selecting the whole length BEFORE putting in the 8va). I had just selected the starting point, and then found that the end point refused to move further than two bars.
The default lengthening key commands work relative to voices for octave lines: if you’ve got cross-stave notation for example, the octave line will get “stuck” on the last note in the same voice that it started in. You should still also be able to lengthen using the alternative command, Ctrl/Cmd-Shift-Right Arrow, to lengthen by the rhythmic grid regardless of voices (although I’m away from Dorico so can’t double-check my memory right now).