In piano, I have 2 half notes in the bass tied over a bar line. It would seem to me that you need only put the 8va sign under the first note, as tied notes are treated as one note and the second note of the tie would stay at the same octave as the first note, but this doesn’t happen as the second note transposes down an octave. Octave lines do take up space, although not a lot but sometimes octave lines and pedal lines can crowd a page and this would be a nice feature to have.
Certainly it would be nice if (in addition to placing the line above or below) it could be hidden for situations like yours.
I don’t think that advisable, because a single 8 under a note without a line can mean add an octave below, not play an octave lower.
John: I don’t really know who would interpret a tied note as a shift in the octave for the second note.
Richard, I think John was replying to my idea rather than commenting on your request.
Ok thanks for the clarification
I meant that putting an 8 under a note without a line following it would mean to add (not shift to) an octave below both the first and the tied notes. Since a single 8va without a line is non-standard even for a single note much less a pair of tied notes, the player might be confuse it with the notation I just described.
There may be composers who don’t use continuation lines at all and simply write 8va and then loco, but this is, in my opinion, inadvisable because players are so accustomed to the continuation line that they thrown off when they don’t see one.