Check out : "Stigma:Tattoing and Branding in Graeco-Roman Antiquity,"
Journal of Roman Studies 77 (1987) 139-155
According to CP Jones in the above article, Romans did in fact practise branding and tattoing, mostly gladiators, slaves and criminals and usually to denote ownership of individual. Amongst free Roman citizens it was most common in military but not an established practise and mostly amongst non-Italian soldiers especially as the Empire grew. Citizens tended to shun tattooing though, and apparently there was a booming trade in tattoo removal.
so yeah, he could have had the SPQR tattoo but unlikely for a general, more common amongst lower ranks. Definately wasn't institutional army procedure though. they make it look like every soldier gets one as soon as he enters army.