Your Op Limits most likely say you must follow FAR 91.205 for night or IFR operations. 91.205 says you must have ‘approved’ position and anti-collision lights for night operations (not for day-ifr though). Elsewhere the fars direct you to run anticollision lights all the time, if you have them (except for safety). Most seem to define ‘approved’ as meeting the performance specs found in the fars. That’s where you will find the angular coverage as well as minimum brightness in various directions. In principle the owner/operator should check this, although most rely on a statement from the manufacturer that the lights are TSO’d or meet the TSO performance specs. I doubt that a mirror will provide the needed coverage and/or brightness.