With the head off, you can try to cut the lip off that retains the electrode (taking care near the combustion chamber). I did this with an old plug to make a compression tester adapter using a hacksaw. A Dremel with a cutoff wheel might be easier in close quarters. The electrode should drop right out, hopefully this would give you something to grip on the inside diameter, perhaps with a fluted easy-out (what a misnomer that is!)??

2000 Mitsubishi Montero Endeavor, "Katsumi", 138k
1993 Toyota Tercel, backup vehicle, 93k
1989 Isuzu Trooper, "Beniko", R.I.P. at 233k

My 'Genuine Isuzu Parts' decals - for sale