Faking a reload while disguised is something that people have "kind've known about" for a while now. I figured I'd clear things up with a demonstration using a mimic bot.
For one, the time you spend actually reloading does not affect the time your disguise appears to be reloading.
The next important thing to note is that not all disguise reloads are equal in believability. A lot of people assume demoman is best, but I actually find it to be sub-par compared to the others.
If I had to rate them from best to worst, I'd say it's in this order:
Engineer
Medic & Spy
Soldier & Demo
Scout
The classes that weren't tested were left out because they don't have reloads on their primary weapons (I know, who'd've thunk it?).
Hope this is helpful to any other spies out there.
~G-Mang