Yup, I think the pop-up doing the repeating is the way to go, the problem is that the SB24/28 don't have the built-in optical slaves (it's why the SB-26 typically goes for more on the used market), and you'd need to add external ones. Hecka lot
cheaper than radio triggers, though.
Or (wild idea), you hook up an optical slave to your Cactus V4, then tape the slave over the pop-up.... that's how some folks use the V4s with their hotshoeless P&S cameras. Bonus, use enough gaffa tape, and no on-axis light!
(I actually don't know if I'm serious about this suggestion).