Phantom power can almost always be switched off. If it can't, then it might be some other kind of powering sloppily labeled as "phantom." And that would be far more likely to harm a ribbon mike than actual phantom powering would be.
One other concern: "Passive" ribbon microphones such as the Fathead put out weak signals. If you translate the Fathead's specifications into modern Western terms, its sensitivity is only 1.2 mV/Pa, while condenser microphones typically put out 10 times that voltage (~20 dB higher signal levels)--and most recording equipment nowadays is designed for those higher voltage levels. Low-cost consumer gear may thus prove too noisy for good results. Of course a lot depends on how loud you play, and where the microphone is relative to your horn. But you're rather more likely to damage a ribbon microphone by direct-miking a brass instrument than you (usually) are with phantom power.
--best regards