When someone says they want complete control over the locations of bits on a drive that could be taken to mean the actual magnetic domains (which aren't simply digital values anyway) or it could just mean the logical digital values that are seen by the OS and filesystem drivers. I wonder if by using phrases like 'magnetic hard drive platter' and mentioning Reed-Solomon encoding, kidcharles has made people think of the lowest level meaning of 'location of bits', whereas in actual fact he just wants something like /dev/sda.
'A free and open source solution' is mentioned.. but theres no mention of what form this solution might take. In the case of a custom HDD controller (which people have suggested as a solution) this would mean something like a VHDL or Verilog implementation (but it would be highly vendor and model specific, plus there'd probably be IP issues). But when people say 'a free and open source solution' they tend to mean software running on a common desktop OS. Oh and I guess there's the question about 'any good utilities' which suggests that kidcharles isn't after an entirely programmatic approach.
So, if I'm correct, any decent disk editor will supply the required level of control. I can't be bothered to look any up (Google will help out here) but I know there are plenty (open source, free or proprietry) for Linux, DOS and Windows. Hell, you could possibly do it all with shell scripting under Linux if you so desired :-)
Alternatively, maybe kidcharles wants to make spirograph like artwork for creatures that can see in the magnetic spectrum.