Example- If Arabidopsis U6 promoter will work in Rice/or Brachipodium

I am not sure whether anyone has done this kind of experiments where AtU6 promoter was used to drive a gene in rice or other monocots.

However, from this company's website (see below link), it seems as if they prefer to use AtU6 in dicots for CRISPR. This is what they say:  "RNA is driven by the U6 promoter from Arabidopsis thaliana (for dicots) or from Oryza sativa (for monocots)."

http://www.dna-cloning.com/

I cant tell you for sure. But, for generating overexpressor/ RNAi lines, I used xylem specific promoter from poplar in Arabidopsis and it worked well. But, I cant say for CRISPR constructs. May be worth going to literature.