how to disarm a potentially pressurized whipping siphon?
I don't know how to tell what the pressure is on a whipper, but from the results of a blocked one, I'm going to make the following suggestion for attempting to safely disarm it:


You need containment.  If you open the container too quickly, you're looking at a massive mess to clean up.  I'd suggest possibly a cardboard box (disposible) or a large plastic container (cleanable), with a clear plastic trashbag over it, so that only your arms are going in.  If you don't have a large clear plastic bag, you'll either need to make a window (we can cut a cardboard box, then tape the bag down) or find something else clear (shower curtain liner?).  Containment also reduces the odds that the head will fly off and damage something.
Goggles or other eye protection wouldn't hurt incase something goes wrong.
Place the containment vessel on its side, with the whipper inside, and the opening so that you can see into it.
If you have assistance, tape the bag around your arms, so you have a good seal.
Slowly open the container ... if it's like the case when my mom did this, a little will leak out after a while.  Give it another partial turn, and some more will leak out ... go slowly.  Suddenly, the whole thing will release, making a complete mess.


If you're like my mom, and insist on attempting to save the whipped cream, you can place a bowl &amp; spoon into the containment with you, and scoop it off as it oozes out the side ... but risk knocking the bowl over when it suddenly pops open on you. 

... 

That being said, if you're not in a rush to open it, I'd let it sit for a day or two, with the nozzle up, (possibly after rapping it hard on the floor a couple of times) then attempt to spray ... if you get a gas release, you might be able to depressure it some before attempting to open it.