You can walk around in the C130, once you're inside. The collisionmodel just wont let you enter. So this can be done my making some sort of "enter cargospace" action. Not the most realistic solution but better than nothing.
Now, I've promised an cargoscript haven't I ?
Here it is. Oh and geolox, I've had the same idea. Those animations are already included in the script

.
cargoscript.sqf
Put this in your c130 init or use it somewhere else.
[this] execVM "cargoscript.sqf";
Features:
- loading/unloading of all landvehicles and boats in c130 cargo, limited only by their dimension
- animated ramps (they open before loading/unloading and close afterwards
- unload in midair and your cargo gets drop with a parachute
- passengers of a vehicle in cargo are moved on a c130 seat when they leave their vehicle while in cargospace (noone will accidently fall out of the plane ;P)
- the vehicle you want to load has to be placed near the ramp
I've not tested this in multiplayer yet and it may need some adjustments for this use. Singleplayer should be fine though.