Google knows I did the 301 redirect internally
What do you mean by "internally" here?
I've got a vague notion you don't really mean 301 redirect at all; you're asking about an internal rewrite. A 301 means "If you ask for a particular URL, the user-agent will be told to ask for something else." No way to camouflage that. But the new URL-- whether in the flesh or in a search engine-- won't have a big red label saying "Formerly known as..."
I think I mean by typing in info:mydomain when looking for 301 redirects
Can you say this in different words? I'm not familiar with "info:" as a search element. Casual experimentation suggests that it gives you content of the "description" meta tag-- if there is one-- for the specified page. Which is interesting in its own right, because I had no idea you could do that, but I don't think it's what you meant.
If you type info:example.com/dir/filename.html and it happens to be a recently redirected page, you'll be shown the new URL-- with description, if any. (I am experimenting here on my own site. Beyond a certain age, it doesn't say anything about the requested URL, even if the redirect itself is still active in htaccess.)
Well. That was all extremely fascinating, but probably had nothing to do with your question.
:: wandering off to investigate why google claims never to have heard of one particular URL ::