I apologise for re-addressing the thread so late...
I'm completely new at this so please bare with me;
Firstly of all I should probably point out that most of what I'm about to write is pure conjecture, based heavily on what I have learnt from watching all of the Stargate franchises and what I know of physics as we understand it today.
I would approach this question first of all from an engineering POV. However, firstly I must highlight a foundation for my assumptions.
I. The Stargate film must be completely ignored... The Roland Emmerich film, although brilliant, was sadly a 90s film with very little true understanding of the physics it was trying to work with. At the point in which they first activate the gate, they send a MALP through and claim it went to a planet in the "Kalium galaxy? - On the other side of the known universe." - I'm sorry, but no... just no...
II. The artificial wormhole created between two stargates is what we understand in physics today to be an Einstein-Rosen bridge. I make this assumption based on information learnt from the franchise about how the gate works.
There is some sort of field at the EH that breaks down matter into it's base molecules in order for the information to be transmitted through the wormhole. - This corresponds to an Einstein-Rosen bridge wherein a wormhole is actually less like a 'tunnel' of equal height in all directions and it actually more like two cones stuck together at there point.
At the thinnest part of the wormhole, ('the middle') only the smallest matter can get through.
III. The EH that we see as a puddle is a 2D projection of a 3D object. Bare with me on this one.
A wormhole is obviously a 3D object it has height, width and depth. However the 'puddle' is a 2D plane in a 3D world (a hole).
IV. We know that information (be it people, machines, food etc.) can be stored (and are stored) in a stargates 'buffer' before being sent to the destination gate. We also know that if something goes wrong a stargate can 'spit' this information back out at the 'dialing' end.
Now, assuming the above information is true. We can take an engineering standpoint and imagine for a moment we are the gate-builders.
As a gate-builder, I know that I can transmit information through my wormhole in both directions. The problem is what happens in the middle (the smallest part of the wormhole where only a single molecule can pass through).
If I sent a Hydrogen atom through the dialling gate at the same time as I sent a hydrogen atom through the destination gate and they met in the middle, due to the speeds they are travelling, they would simple annihilate. Making all gate travel in both directions impossible.
So, as a built-in, hardcoded, safety feature I would program my stargate to be only one way. One information stream sent from one end, to the other end. Furthermore, the dialling end would be the departure point as that gate has chosen the destination.
A gate could potentially send travellers in both directions like so: a man walks into the dialling end, is 'dematerialised' and stored in the buffer. At the same time a woman walks into the destination end and is 'dematerialised' and stored in the buffer. The dialling stargate sends a signal (through the wormhole) to the destination gate asking if there is a complete pattern stored? If the destination gate responds with 'YES' the man's pattern is sent and on completion the woman's pattern is then sent. - This would eliminate the inhalation problem in the 'middle' of the wormhole. Now the transaction is complete, they can re-materialise each individual at there opposite ends. The only remaining problem is that a gate is probably unable to to perform de-materialisation and re-materialisation at the same time.
My second safety feature would be to make sure that the field that de-materialises information only act's on matter from the front. I would do this by checking for attenuation of the field and only starting the de-materialisation process once force was applied from the 'front' of the gate. I would ensure this was the case because I'm not entirely sure that de-materialising two different pieces of information at the same point in space and time is at all possible.
The EH is a 2D (hole) in 3D space, walking into the back of it would do nothing, because technically, it isn't there... So the only thing you would actually be doing is walking through the wrong end of a de-materialisation field. Which (because of the second safety feature) would do nothing... Unless you wanted to program it to repel anything that exerted force onto the field. Which brings us to the third safety feature...
I would, most certainly program the 'back-end' of the field to repel matter. This doesn't mean it wouldn't 'throw you off' it would probably simple feel solid. This would be because if you could just walk through the back of an active stargate and come out at the front, completely un-touched so could a projectile (like a bullet). If which case your journey through a stargate would always have the potential risk of dying immediately on re-materialisation.
Please pick holes in my suggestion so that we can get to the bottom of this together
TL,DR: If the 'ancients' are clever, you physically can not walk into the back of a stargate. However, if you could, you would just walk straight through and nothing would happen to you...