Are there any ashkenazi poskim that do not allow wigs as a hair covering for women?
ב"ה Shalom Rav Ya'akov Emden (שאילת יעבץ חלק א סי' ט) said it it is forbidden. Also the Tifferet Yisrael on the mishna (in Shabbat 6:5) explains that going out with a wig which is mentioned in the Mishna is referring only to a private domain not a public place. See also Maharil Diskin (שו"ת קונטרס אחרון רי"ג)For more references see Otzar Haposkim (Even Haezer 21:2:5). However, the Mishna Brura (75:15) allows to wear a wig as a head covering where the custom is to allow them. Shana tova.