I just finished weaving and hemming a set of huck spot dishtowels, which I wove in 8/2 cotton sett at 16 epi/ppi. They look terrific. I had no problem maintaining that beat (I use a firm tap, not a light kiss or a hard whack with my beater). I imagine it would be difficult for me to get 20 ppi on a 20 epi sett, as I struggle personally to even get 18 epi/ppi with 8/2 cotton. I'm seriously considering 16 epi/ppi as my new personal sett for plainweave.
Before you go for the huge yarn eating curtain project, maybe try a run of towels at 16 or 18 epi/ppi in the lace weave of your choice. See how you like the fabric wet finished. Then you're not risking a large yarn and time investment in making curtains you later might regret. Aren't we always told to sample, sample, sample? Towels are the perfect medium for that sampling!
Both Bronson lace and allover huck spots are amazing at showing off the lace by filtering light. You'll be happy with either structure.
One additional thing to consider is that you can use plain weave at the selvedges, and for the rod pocket, if you're doing huck spots or Bronson lace. I like a quarter inch or so of plainweave at the selvedges just for smooth edges, and to avoid having to use floating selvedges. Nothing against floating selvedges, it's just that I like a narrow plainweave selvedge with lace structures. This is something else you can test out in a dishtowel warp.
Have fun designing your project!