Bilirubin metabolism and UGT1A1 inhibition in human vs. monkey?
UGT1A1 is common among most mammals that produce bilirubin (rats too). I haven't seen any evidence to suggest an alternate pathway in mammals of bilirubin metabolism. Look into gilbert's syndrome patients (they have a SNP which is a mutuation that causes UGT1A1 to be less efficient and thus a rise in plasma unconjugated bilirubin).

Also, bilirubin analysis from plasma/tissues is fairly straight forward with HPLC or LCMS assays. UGT1A1 inhibition in vitro however, is still a challenge. I think the trick will be to use CRISPR to knock it out.