Actually, couldnt mechandrites be hidden under the robes (coiled around the body/limbs)? And for face, I swear Mechanicus Factor has some sort of prosthetic face, and Radical´s Handbok has bionic called Twitch-Mask, which is especially made for masking? Much will depend on qualitty of bionics in question (I think it is one of the most common perks of Best Quality). I would be more concerned with masking the heat/energy signature.

Or go completly other way and mask your Mechanicus character as servitor. Ordained techpriests could have some theological issues with it, but Skiitaris, tech-dabblers or Infiltraitors would be more accomodating.

Factors have Fabricated Flesh, masking their cyber-mantle and also making other "upgrades" to be natural or at least common augmetics. Plus, a lot of other groups wear masks all the time as well, the Templar Calix or Janissaries (from Calixis fluff at least).

For most others, the player who is the tech-priest just needs to have a story ready for whatever identity they are trying to go undercover as.

~ alemander