Pound a lot of liquid IV or similar electrolyte supplement. You should be able to dilute enough to fly under the detection thresholds but still have enough vitamin/mineral content that it doesn’t get rejected. Had this happen to someone I know who was smoking daily and had a prescription for an amphetamine ADD med, neither showed on their test results.
…Signal’s never had federation, though. That’s been one of the biggest sources of criticism toward Moxie’s design decisions for the platform.