Thanks for the answers.
In particular, is the WIF that begins with: "L1111111111......." a valid private key? Leaving out all the checksum bytes of course so those can be anything. I just want to know if it's valid to place a sequence of 1's inside a private key WIF (I read somewhere that that may not be the case for addresses), as the number "1" in base58 is equal to the number "0" in base10.
Of course. It is just a text in given encoding. It is only our (people) ability to find patterns which looks "nice"...
WIF L111111111111111111111111111111111111111111111CbeH6a is fully correct, leads to private key
70cfa0d40309798a5bd144a396478b5b5ae3305b7413601b18767666bdb2f800
which is key like any other...
Some strings looks "nice" in binary, others in hex and others in base58 encoding....