Reading things for about the billionth time.
Ready is a Standard Action, a preparatory action for another action you take later and resets your initiative. That later action is what can be a standard, move, swift or free action.
The preparatory or special initiative action in this case is 'set against a charge'
Whether this act is obvious or not is completely open as far as I can tell to the GM's judgement and however he wishes to handle it.
The encounter starts and initiative is rolled/decided.
Our OP is already aware and not surprised (In fact it sounds as if he is deliberately setting an ambush). Any rolls and or decisions about the guards awareness of what lies ahead them are made. If I were the GM I'd probably wrap this into one contested roll with benchmarks to decide just how much they note about the situation (guy outside door, guy outside door with weapon available, guy with weapon who might be trying to be all sneaky like and poke someone unexpectedly etc.).
So ... Our OP uses a Standard Action called Ready (set against a charge) and this may or may not be obvious subject per GM discretion. He does so at minimum as his Standard Action taken during the surprise round of combat. Note that any aware guards may or may not go ahead of him in the surprise round. Any aware guards that win initiative and I'm thinking the planned ambush/Readied action goes up in smoke as the aware guard(s) shout warnings. If none of them go ahead of him then continue:
Unaware (surprised) guards continue Running and he impales the first idiot to foolishly rush into the situation. (double damage for braced weapon)
Ditto for Aware (not surprised but slower reacting) guards who lost initiative.
Now after everyone has finished the surprise round the first round of combat starts. Any unlucky folks who go after our OP might now get struck again by Mr Tricksy and his reach weapon (Sneak damage vs anyone who lost initiative and is therefore still flat footed).