Hmmm - I'm not so sure about that. The reason I said "does not exceed the frontage of the number of elements in ambush" is because if a unit of plashing has no ambushing elements behind it then I don't think (just my opinion) it is part of the ambush. There may be an exception in the rules that I'm not aware of, but a quick read hasn't turned up anything. If you wanted to conceal plashing with no hidden elements behind it then it may be counted towards the 8 (or 16) elements you can hide in ambush....?
How about this? Lets say your opponent deployed a unit of plashing with nothing behind and you wander into it inside a wood (starting from outside 40 paces). The TF has no combat factor so I presume it does not fight. Plashing is defined as not being a hidden obstacle so troops who crossed it (assuming they can cross it - see next sentence) do not make a spontaneous advance. Under the 'storming fortifications' heading (page 42) it says "Mounted troops can only assault TF". Assuming you aren't sending your mounted into the wood (it would be mad to unless you used a Guide stratagem) and it was foot that contacted the TF, does this mean the foot cannot fight the TF? Is the plashing simply removed or must the foot move around the TF?
On a side note we played the Battle Cry tournament in Auckland over the weekend, and in the first round I used the ambush stratagem against EI Romans. A column of AxS came wandering into the woods in a single column only to be jumped on by my PsO, flanked on one side and a flank wrapped on the other - BOOM! 2 dead AxS in the first round of combat. Very nice
I wouldn't normally fight Ax with Ps but the Ax never got a chance to fight the Ps on their terms given a flank march arrived on that very flank the next bound. Although the game deservedly ended in a draw .....