Could processed meat be made less carcinogenic with celery juice or other fermented vegetable broths enriched with nitrite?

No. The source of nitrites is irrelevant in the formation of nitrosamines and N-nitroso compounds.

“Natural” sources of nitrate, such as celery juice or spinach extract, contain high concentrations of nitrate which has to be processed to reduce this to nitrite for use in the meat product.

Also, adding nitrites from vegetable extracts for a technological function and/or as a preservative, is banned by the European Union.