I've just recruited Cass, but I could find no option in her dialogue that would get her to say that she wants to pay her last respects to her destroyed caravan. Any idea what could trigger it?

Current game situation is as follows:

I've independently discovered the site of Cass' destroyed caravan before I recruited her.

I've completed You Can Depend On Me before I recruited her.

I have Birds of a Feather active, and I've been told to bring Cass to the Silver Rush.

Bringing Cass to her caravan does NOT trigger her desire to visit it or advance Heartache by the Number in any way.

did you talk to her? when i have her as a sidekick, she usually starts talking about what happen to her in a short period of time. did you take her to the new vegas area yet?

While walking around, you need to talk to her and bring up the dialgue option. Ask her all of the possible questions and it will give you the Quest. GL.

I've sorted it out. Was a karma issue, apparently. Once I've killed half a dozen fiends, Cass warmed up and told me about the caravan.

Yeah, if you are at negative karma she will get all pissy, and if you stay there and keep talking to her she will leave. If you are at >= 0 karma she will give the quest immediately upon opening dialogue with her, regardless of how many of the subsequent locations you've already been to or if you have done any other related quest(s).

In fact, I make a habit of visiting Durable Dunn's Sacked Caravan early on to get the Combat Armor and weapons to fix up and sell.