I was discussing the Ashkenazi minhagim and hanhagot of Tish'a beAv with a contact of mine, namely the universal Ashkenazi practice to refrain from tallit and tefillin until after chatzot. He told me that some among the Ba'alei Tosafot had sought to entirely eliminate the wearing of these on 9 Av, due to the magnitude of the mourning associated with the day.
My question is twofold:
- What is the source in Ashkenazi Rishonic writing for this opinion ?
- What justification could be applied to make this practice not be bittul 'aseh (nullification of a positive commandment) ?