Thanks for the suggestions. I should buy a set of left-handed drill bits someday.

I got them out, but not without a struggle. Tamper-proof, my ass

First I tried to get a 1/16" welding rod to stick (usually the opposite of what I'm trying to do!), but ended up gouging a small pit in the plug instead.
It had gotten hot enough that the temper was gone and I could drill into it with a #36 (picked that size in case I had to tap for a screw thread) deep enough for an easy-out.

For once, an easy-out actually worked. Turns out they aren't welch plugs but a piece of solid steel rod about 1/4" deep.
Now I'm going to cut a small screwdriver slot in the heads of the needles with a Dremel cutoff wheel so I don't have to use an Allen wrench and hope it doesn't strip out. Then it all goes back together.

Here's the "before" picture of one of them for TJP (The secondaries have the bosses also, but they're not drilled. Just a "dribble hole" in the venturi).