if one forgot to say asher yatzar and then said hamapil should he say Asher yatzar, ( I don’t see a צד why he can’t)
Yes he should say asher yotzar. This that the Shulchan Aruch writes that we don’t talk after hamapil is only for things that are not important, but for things that are needed, it is permitted to talk out. Therefore it is permitted to say asher yotzar.
Pri Hasadeh 1-93, Hisorirus Teshuva 128, Taharas Yom Tov 13 pg. 188, Aruch Hashulchan 139-6, Margenisa Tava pg. 303, Emes L’yackov ftnt. 254, Shearim Hamitzuyanim B’halacha 71-4 (seif katan 2), Be’er Moshe 1-63, Tzitz Eliezer 7-27 (3), Birur Halacha O:CH 2-pg. 261, and. However, in sefer Imrei Yosher (Nezikin 98) he writes in the name of Chazon Ish not to say it, but see Birur Halacha (Tlisa’a) pg. 132 who brings the Chazon Ish, but brings numerous poskim that it should be said, even according to the Mishna Berura.