Workaround 2 is almost right, but the crotchet spacer s4 (or a hidden note) will throw out the timing (as can been seen in the following measure).

Instead, start the phrasing slur in the second volta on an empty chord <>:

\alternative {
    \volta 1 {
        c2 c2\) |
    }
    \volta 2 {
        \shape #'((-2 . 1.4)(-2 . 0.6)(0 . 0)(0 . 0)) PhrasingSlur
        <>\( c2 c2\) |
    }
}