Is unary machine code a concept?
Yes, you could express machine code in unary.  How do I know?  Well, anything that can be represented in binary, can be represented in unary.  Machine code can be represented in binary (as a binary string of bits)... therefore it can be represented in unary (as a unary string).
It wouldn't be useful or practical to do so.  It's not just less comfortable; it's much worse than that.  Conversion to unary involves an exponential increase in size (a string that takes $n$ bits in binary would take $2^n$ symbols in unary), so it would be totally infeasible in practice.  But in theory, you can imagine it; there's no barrier in principle.