My conclusion so far:
c2sc feature: sub Germandbls by germandbls.sc;
smcp feature: sub germandbls by s.sc s.sc;
Otherwise, no mention of germandbls.sc/Germandbls is strictly needed.
But, and this brings me to my original question: IF a designer wants to use either germandbls.sc or Germandbls (note the cap G), they have to do so through the glyph panel, which seems cumbersome. Could it be helpful to include both as discretionary ligatures? Or would that just be confusing? I mean, in essence, it kind of IS a ligature, and it IS discretionary, right?