I've been wondering about responding to past issues also. If you know you want to respond but you don't have time you can always respond and say something like, "Remind me to tell you something about this later." This would keep the link going anyway.

If we're browsing through old volumes and want to respond I guess we have to start a new line and include the volume and card number of the old card.

[Editor's note: EXACTLY! You can now do this quite easily by marking the card and then inserting the reference with the tab key; See Vol 4 John 28]