The standard choice for MMA processing is actually stainless steel, because carbon steel/iron/etc scavenge the inhibitor used to prevent thermal runaway.
HDPE would work, and often how it is transported in drums, but for actual processing, everything would normally be spec'd as stainless steel.
Which, of course, is pretty spark resistant to begin with.
Even if this wasn't true, this is not a hard problem, you can use non-sparking tools, proper coolant, lots of things to avoid sparks.
Or you know, we could require that highly flammable materials subject to thermal runaway have "drill here in case of emergency" patch of non-sparking material or something.
The cost of ATEX/Class 1 Div 1 compliance would not really go up if you required this.
Inb4: I don't work in the industry, my knowledge is limited to a faint memory of a college course on fluid dynamics.
If I recall correctly, high pressure ignitable stuff can spontaneously turn !!FUN!! in absence of heat if it is suddenly relieved through a pinhole. Basically jet is followed by a ring-like zone where the stuff mixes with oxygen. Jet creates tiny zones of very high temp, thus igniting the mixture ring that follows.
Maybe the thought process is that leak would effectively be pressure normalizing over a larger area and more gradually than creating a tiny hole, so whatever I said about jet ignition would not be applicable
So MMA has a fairly high autoignition temperature - it's >815F (gasoline is like 475F).
It also has a fairly small explosive concentration range (2-12%).
(the inhibitors it gets mixed with are about the same autoignition temperature, but have higher flashpoints).
In any case:
1. The pinhole issue can be resolved pretty easily by making a big hole instead of a small one :)
2. MMA is normally stored at ~0psi. The increasing pressure here is from runaway polymerization boiling the liquid (the runaway comes from the polymerization being strongly exothermic). These tanks are not built to handle very much pressure, usually a few psi max. I doubt whatever the tank pressure is currently qualifies as the kind of high pressure you are talking about.
3. There is apparently a significant crack now that is relieving most of the pressure. Which is preventing it from bursting. Assuming the crack didn't exist before this all occurred (so far the prevailing theory is it did not, but who knows how it will turn out), it almost certainly started as a pinhole that propagated, so if it was going to ignite the way you said, it would have happened already. My guess - even if what you say is 100% correct, the pressure is never high enough to cause issues, even with pinholes.
Which, of course, is pretty spark resistant to begin with.
Even if this wasn't true, this is not a hard problem, you can use non-sparking tools, proper coolant, lots of things to avoid sparks.
Or you know, we could require that highly flammable materials subject to thermal runaway have "drill here in case of emergency" patch of non-sparking material or something.
The cost of ATEX/Class 1 Div 1 compliance would not really go up if you required this.