I note that this is possible by army list special rule - for example to allow Bill to interpenetrate a unit of longbow and move in front of them..
Is there a reason that heavy foot cannot generally interpenetrate other foot  - but more specifically, loose foot?

I watched a game of ADLG the other day and saw that they were doing this, then wondered at not being able to do the same in MeG, hence the question.

Did you have any examples in mind where this happened?

MeG follows historical precedent for interpenetration rather than just an arbitrary rule. Also some of the mixed formations simulate some of these interpenetrations

No I don't have examples, I know we have a significant number of knowledgeable people involved in MeG and just wanted to ask the question..

I'm not sure what the historical precedent is, or is not. I know Romans could switch ranks. I've seen it done on movies. Otherwise, I don't have any idea, other than knowing it's allowed in other games.

Interpenetration of anything except LF is very difficult to do. 
Even that isn't so easy in reality.
I doubt there is any evidence much to support more than we allow in MeG.

S