I've been semi keeping up with this thread and thought I would chime in. I've tried something slightly different to avoid frizz when diffusing. Since I don't have a hooded drier (maybe one day), this is what I did before diffusing...after applying Stylers and such, with my head upright, I held my dryer over my hair (with diffuser attachment) and I slowly dried my hair from top to bottom (without touching my curls) for 5-10 minutes, just enough to set my hair. Then I began to pixie diffuse to about 85% dry. This give me less frizz than I've had before when just pixie diffusing. I've always tried to not move my hair around a lot when drying/diffusing since it causes some frizz. I just thought I would add this little trick that I learned. I hope that wasn't confusing.
So, essentially I tried to semi-duplicate the effect of a hooded dryer (since I don't have one), to set my curls before diffusing. This worked for me and maybe it will help someone else.