The solution was easy ie. reverse can be made with PWM_AUX_REVx params - a bit confusing naming for those (PWM_MAIN_REVx would be more logical)…
for my Airframe i reversed PWM_MAIN_REV1/PWM_MAIN_REV2 and switched the servo plugs too.
so against the general rule for Flying Wing the left aileron is pluged with MAIN2.
my Airframe: E-flite Opterra 1.2m, Futaba S-BUS connection, Pihawk4 mini with Firmware Version v1.10.1