I'd like to install a subpanel using a 2-pole Square D QO GFCI breaker in the main panel and then installing individual QO CAFCI breakers in the subpanel where needed for the dual protection. It seems more cost effective than buying 12 new dual GFCI/CAFCI breakers for each circuit. Anybody try this?
Yes, a GFCI sub-panel with AFCI breakers sounds like an excellent way to do that. It avoids the European problem of having the whole house GFCI (RCBO) trip and ruin a refrigerator full of food and freeze pipes (though some European furnaces don't require electricity).
I don't see a problem daisy-chaining since they are different protections. I also like having the arc-fault protection more local than the ground-fault. The GFCI doesn't care, but the AFCI does, because it is literally listening to electrical noise, and it's easier to "hear" when fewer things are talking.
The only thing you need to watch is that your large, full-panel GFCI has the sensitivity that is legally required for bathroom and kitchen circuits. Some of the Euro-style whole-house RCBOs have 20-30ma thresholds, which is not sensitive enough by far.