Traditional grammar says that to be is followed by a complement in the nominative case, not by an object:
It is I.
In everyday usage, the vast majority of people say "me":
It is me.
Both are correct, but "It is I" sounds very formal.
"This is I at work" sounds ridiculous, though. It is so unidiomatic that it should probably be considered incorrect. So it seems that the nominative forms are only really acceptable in a subset of sentence types.
In conclusion: We would always say "This is me at work".
Addendum: Cambridge has these examples:
The dog barked before we even came to the door. He knew it was us.
He knew it was we.
You know the girl I was telling you about? Well, that’s her over there.
Well, that’s she over there.
But also points out:
In some formal contexts we use the subject pronoun (e.g. I, he, they, we):
I can’t exactly remember whether it was he who asked me to go with them. (less formal: … whether it was him who …)