Disk brakes exert masses of force on the frame, adding a strut just transfers the force from one stay that wasn't designed to take it to another.
Back in the day when disk's were new people did just add tabs to existing frame designs, they either fell off altogether or ripped the stays out of the dropouts.
Maybe try something like a formula mount which uses the calliper as a brace between the stays (used on the old sunn frames mentioned above which were lovely).