Question: We have the minhag not to eat matza between Purim and Pesach. Can we use a matza for lechem mishne if our minhag is not to eat it?
Answer: Rambam (Chametz Umatza 6:12) writes that one must not eat matza on erev Pesach. Later poskim, including the Mishna Berura (471:12) note that some have the minhag not to eat matza from Rosh Chodesh Nissan. R’ Moshe Feinstein (Igros Moshe OC 1:155) notes that some abstain from eating matza for 30 days before Pesach as according to one view in the Gemara (Pesachim 6a) this is when our Pesach preparations begin.
R’ Eliezer Waldenberg (Tzitz Eliezer 11:23) quotes the Pri Megadim (Mishbetzos Zahav 274:2) who writes that one who is particular not to eat pas akum, bread baked by a non-Jewish baker, can still use such bread for lechem mishne. So too, one may use matza for one’s lechem mishne even on erev Pesach. Additionally, even if one cannot eat the second bread, it still serves to remind us of the double portion of mann.
R’ Betzalel Stern (Betzel Hachachma 3:110) quotes R’ Chaim Zvi Hirsch Manheimer (Ein Habedolach 61) who writes that one who does not eat chadash cannot use bread made from chadash flour as lechem mishne. Accordingly, one would not be able to use matza at a time that one does not eat it. Nonetheless, he disagrees, writing that one may use a second bread or lechem mishne even if they cannot eat it. Thus, even though one should only eat shemura matza on seder night one may use non-shemura matza for lechem mishne (See Chelkas Yaakov 1:95:2; Kaf Hachaim OC274:14; Rivevos Ephraim 1:202).
R’ Ovadia Yosef (Yechave Daas 1:91:13) notes that usually food that cannot be eaten would be muktze on Shabbos. Nonetheless, as one can feed matza to little children even on erev Pesach, it is not muktze.
In conclusion, there is no issue with one using matza for one’s lechem mishne before Pesach, regardless of one’s minhag.