not ANY TX power... only up to its directivity if inserted the correct way around. I would guess maybe 20dB (?) in reverse direction. If you add another 20dB as plain attenuator (which a LNB with its huge gain might easily compensate) you have 40dB of protection for the LNB again accidental TX.
Great and thank you for the pointers. I will look into this alternative too.
73, Sion, 9M2CQC