Pewter,
I am not a herald, but here’s my thoughts...=)
In the Kingdom of the West, the difference between Renaissance Faire and SCA names is that Faire names tend to be more on the fanciful side, while SCA names tend to be more plausible names for the time period. (It’s not necessary to be documented that somebody did have the name, but at least prove that it’s plausible for somebody to have had it.)
With that in mind, your suggestions of "Peeter Thisblewhat" and "Harold Scrotum" tend to fall more on the fanciful side. While still somewhat commonly used in the SCA, I doubt that the Heralds would approve of such a name for "official" use.
My understanding is that Pewter, while probably not a common given name, is a period material.
Remembering that last/family names of the period were often descriptive and given based on job, location, etc...
Perhaps your back-story could be that you grew up in a town with two Michael Smiths. You’re one of them. (You could choose any name besides Michael Smith if you wanted, but say that there were two of you in town.) The other Michael Smith is the town Blacksmith; you are the town Pewtersmith. (Hence the last name Smith.) In order to tell the difference, the townsfolk decided to give you the nickname of "Pewter".
Any lurking heralds on here want to comment?
Michael