The host end needs to look good while the keyboard already has a 5 pin connector and space for the resistor and switch, so unless you know of any reason why the OTG connection needs to be right near the host this seems like the way to go. At worst I end up with a slightly thicker than normal cable but it should still work like any other as nothing is expecting the extra connection, if I'm understanding it right (OTG didn't exists when I did USB for my dissertation at uni...)
Please let me know how I can order this cable and I will do so