I did confess to splitting hairs... but I guess one has to define 'rarely' and also 'in-house' ... and I suppose I'm lumping some Toonami stuff in there, too, cuz it is Williams Street after all and it did leak into the Swim as well. In addition to Tae-te-Swim, Kamal Stoddard, Tim Meany and DJ Clarknova, I would consider everything by Joe Boyd Vigil, Chris Devoe, Cee-lo Green, Dangermouse, Michael Kohler (credited alongside Sean Akins on the First Aid bumps) and The Adult Swim Singers all in-house. Some employees, others sub-contractors or freelance but working closely with WS. Jason DeMarco (Clarknova) tweeted something around the time of all this buzz that made me wonder if he was getting nagged about it, but I never looked into it. I have little doubt that the recent humming of the Jeopardy theme was hummed by an employee, but no one is talking... we know that AssProd whispered the Gigantor theme, and there's a bump of footsteps they claim to have recorded around here somewhere.
So is it unusual? Sure, but not unheard of... and I totally agreed with your assessment of someone whose full time job is music often being involved.