May one interrupt Pesuke Dezimra?
As mentioned previously, the Shulhan Aruch (O.H. 51:1) writes that Pesuke Dezimra begins with the blessing of Baruch She’amar and ends with the blessing of Yishtbach. The Magen Avraham (M.A., O.H. 51) says that once one completes the blessing of Baruch She’amar, one is not permitted to make any interruptions during the entire Pesuke Dezimra. Nevertheless, the Magen Avraham continues, one is permitted to make an interruption, such as answering “Amen” to a fellow’s blessing, in the middle of Baruch She’amar itself or in the middle of Yishtabah itself.
Regarding interruptions within Pesuke Dezimra, there is a prominent disagreement between Rabbi Ovadia Yosef and Rabbi Shalom Messas. Rabbi Shalom Messas (Tevuot Shamesh, Orah Haim, § 12) posits that within the blessing of Baruch She’amar itself, one would be able to make an interruption, especially in light of the fact that the source of the blessing is not from the Gemara but rather from the Geonim. However, he holds that during the actual Mizmorim of Pesuke Dezimra, one is only permitted to answer Kedusha, the first few words of Modim and the first five “Amens” of Kaddish. According to his view, one is not permitted to answer “Amen” to any other blessing or anything after the first five “Amens” of Kaddish.
On the other hand, Rabbi Ovadia Yosef (approbation to Tevuot Shamesh) disagrees and says that during Pesuke Dezimra, one may answer “Amen” to any blessing. He cites the HIDA (Kesher Gudal, § 7:34), who bases himself on the Arizal (Sha’ar Kavanot, pg 1), who follows this opinion. Furthermore, the Ben Ish Hai (Parashat Vayigash, § 10) and the Mishna Berura (M.B., O.H. 51:8) concur.
This disagreement does not appear to be a matter of custom, but rather a novel opinion on the part of Rabbi Shalom Messas. As a matter of fact, in his correspondence with Rabbi Shalom Messas, Rabbi Baruch Avraham Toledano (not to be confused with the more well-known Rabbi [Raphael] Baruch Toledano), who was also from Meknes, disagrees with his approach.
Another practical application is if one must uses the restroom during Pesuke Dezimra. Rabbi Ovadia Yosef (Yabia Omer, vol. IX, § 108:31) rules that one is permitted to recite “Asher Yatzar” during Pesuke Dezimra. The Kaf Hahaim (K.H., O.H. 51:28), the Haye Adam and Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 5, § 2), on the other hand, say that one should wait till after Yishtabah in order to recite it. As a general rule, one should try and wait until after Yishtabah to recite Asher Yatzar, but if one is concerned lest one forgets to recite it altogether, one may do so during Pesuke Dezimra.
Summary: One may answer “Amen” to any blessing during Pesuke Dezimra, in addition to Kedusha and the beginning of Modim. If one must recite “Asher Yatzar”, one should try and wait till after Yishtabah, but if necessary, one may do so in the middle of Pesuke Dezimra.