After one answers Barechu, is it permitted to bless Asher Yatzar even though he is in the middle of the blessings of Shema, if he suspects that he will need the facilities again (e.g., one who drank a lot of coffee) and will lose the bracha? Could we say that it is a passing mitzvah, like blessing on thunder and lighting which some poskim permit during the blessings of krias shema?
Logic dictates that just like one can say a blessing on a passing mitzvah, similarly if he suspects that he will need the facilities again and will lose the bracha can recite it bein haprokim. Also even if he will not need to go a second time but fears that he will forget to say it after Shmonah Esreh he should say it in between the blessings of Krias Shema.
O:CH 66 Mishnah Berurah 19 & 23