I recently had a similar problem with my Mazdaspeed 3 (2010). Driving along and suddenly the clutch pedal has 0 pressure and just springs to the floor when touched. The clutch was engaged, so i could not change gears when the engine was running.. (well you could slam shift it if really necessary I suppose.) Went into gear when the car was off. Anyway, after some troubleshooting and ripping the drivers side of the engine compartment apart to be able to see anything, I didn't find anything wrong. Pressure tests seemed fine. Tried bleeding the line with a pump.. anyway to make a long story short, it turns out the push rod had broken off from the clip that connects it to the clutch pedal. Sadly, I can't seem to locate the pushrod separately, so I'll have to replace the entire clutch master cylinder to fix it.
Why the hell is the push rod made of plastic. Great spot to cheap out :/
Anyway, I couldn't find anything on the web regarding similar issues. So I thought I would add to this thread another feasible explanation to the issue for the next poor bloke this happens to, lol.