I'm trying to apply soft body sim to a Manuel Bastioni lab human and all I can get as a result is a collapse of the body like cloth or explosion of everything.
How can I simulate the output human as it is a balloon filled with gel/water?
It's difficult to get convincing 'outward pressure' in a soft body but one trick is to simply use the 'bending' spring force to make the mesh attempt to retain its shape. In addition, try reducing the Mass of the soft body and significantly reduce the Gravity weight so as to make it 'float'.
Remove any modifiers that are no longer required (such as the default armature modifiers). Also, the default Manuel Bastioni Lab mesh includes a lot of unnecessary detail around the face and hands that will significantly slow down the simulation - and close-proximity vertices (especially around the fingers) can result in very strong repulsion forces if you enable 'self collision' - so best to either not use self collision or use 'Remove Doubles' on the mesh in Edit mode to cut down the detail a bit (this will potentially join adjacent fingers so use with care - possibly separate the fingers a bit first).
This can produce the following result :
The soft body settings for this were as follows :
Note that the only changes from the defaults are to reduce the mass to 0.5, disable the Goal, increase edge Pull to 0.9, increase Bending to 5.0, reduce Gravity to 0.05.
Increase the Bending spring force even further for a more rigid (effectively more inflated) effect.