Can the guanidinium ion give stable N-nitrosamines under mild metalworking conditions?
By definition, N-nitrosamines are aliphatic or aromatic derivatives of secondary amines, which have a nitroso group ($\ce{–NO}$) attached to nitrogen. In general, N-nitroso compounds are usually formed by the interaction of a nitrosating agent ($\ce{NOX}$) derived from either nitrite salts or nitrogen oxides with an amino substance such as secondary and tertiary amines, secondary and tertiary amides, peptides, substituted ureas, guanidines, and urethanes. Some important N-nitroso compounds are shown below:

As structures indicated, resonance in the framework has nothing to do with $\ce{-N-N=O}$ function (e.g., 1-Methyl-3-nitro-1-nitrosoguanidine is stable at room temperature and commercially available). Nonetheless, guanidine derivatives interact with $\ce{NOX}$ to give stable nitroso-guanidine derivatives (which are type of N-nitrosamides). With $\ce{NOX}$ derived from nitrite salts, the formation of N-nitroso derivatives of guanidine rarely occurs above $\mathrm{pH \ 4}$, but increases steadily with increasing acidity i.e. decreasing $\mathrm{pH}$, a maximum at  $\mathrm{pH \ 2.5-3.5}$ where the yield is inversely proportional to the basicity of the amino substance  (Ref.1).

On the other hand, guanidines do not react with $\ce{NOX}$ derived from nitrogen oxides in aqueous media (Ref.2,3). Note that $\ce{NOX}$ derived from nitrogen oxides worked best with strongly basic amino substances (e.g., aliphatic amines and heterocyclic amines) under non-aqueous or neutral and alkaline aqueous conditions (Ref.2,3).

References:

Sidney S. Mirvish, &quot;Formation of N-nitroso compounds: Chemistry, kinetics, and in vivo occurrence,&quot; Toxicology and Applied Pharmacology 1975, 31(3), 325-351 (DOI: https://doi.org/10.1016/0041-008X(75)90255-0).
B. C. Challis, &quot;Nutrition and nitrosamine formation,&quot; Proceedings of the Nutrition Society 1985, 44(1), 95-100 (DOI: https://doi.org/10.1079/PNS19850015) (PDF).
Brian C. Challis,  Soterios A. Kyrtopoulos, &quot;The chemistry of nitroso-compounds. Part 11. Nitrosation of amines by the two-phase interaction of amines in solution with gaseous oxides of nitrogen,&quot; J. Chem. Soc., Perkin Trans. 1 1979, 299-304 (DOI: https://doi.org/10.1039/P19790000299).