I'm still not getting it.
Why would you make your window frame invisible?
Not the window frame.
You would place the window brick which normally would have no collision.
This would give the visual effect of the window.
Then you would place normal (5X height let's say) brick inside of the window frame that would keep collision, but rendering would be turned off.
It would appear that the window brick was keeping you from going through, when in fact there was an invisible brick inside of it.
Pretty Simple.
Another solution.
Would it be possible to make a thin water brick with visible sides and non-undulo?
If this was the case, you would only need to turn on collision.
Profit.