I am sure someone here will know the answer better then I
But I would say yes, it just will not look as good as 100% because it will only adhere to the poly and not the cotton
I have never done anything other than 100% poly myself but a person I know did it on 50/50 and said it didn't wash as well and another guy did it on 50/50 and it felt like a transfer, (he swears it was sublimation but I swear it felt like a transfer so I am not sure if he knew what he was talking about even though he does do sublimation on mugs) I just can't imagine sublimation would feel like a transfer even on 50/50 but I could be wrong.
Good luck and congrats on your REAL customer
