Removing the rear brake drums on a 544 takes some determination. This might help.
I drilled a small hole in the drum hub just above the key and as close to the drum as possible. For a week every day I filled the hole with penetrating oil. It seeps in, follows the key and I think it helps.
I then bought a big bad puller with a cross bar that you hit with a large hammer. The first one came off with the puller alone, but the other side had to be heated with a torch to get it to pop.
put the nut on flush with end of the axle to contain the drum when it pops and to kind of chase the axle threads when you remove it.
don't give up if it won't come off the first try. Let it cool, oil it again and come back the next day with a bigger hammer and hotter torch.

man, this is not something I would do. Firstly, cast iron is brittle. I'd be afraid of weakening with a hole anywhere. Secondly, I'd be afraid of any residue from the penetrant upon re-installation. Tapered metal on metal must be clean (think drill press or milling machine chucks)

The key should only be a safety precaution. if the drum slips, it will make the key hourglass shaped, and spread the slots in both the drum and the axle, effectively ruining both.

I expect the small hole is okay. But man, be sure to clean the tapered surfaces very thoroughly before reassembly.

The puller you have sounds like the right tool, and it will work without penetrating lube. It might not be easy. I've beat on those things for hours, to no avail, left them under tension overnight... only to be awakened by what sounded like a gunshot at two in the morning, likely due to temperature change.
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-Matt I ♥ my ♂

I'd tend to agree with Matt's thoughts regarding lubing or not lubing the tapered surface. I think the theory here is that the woodruff key is mostly just for assembly alignment (and as a fail safe), and the nut is just there to pull the tapered drum onto the tapered axle shaft into an interference fit, the one stretched onto the other. Once installed like that, you should be able to take the key out (if there was room) and take the nut off, and drive around for years. The drum being held on merely by that interference fit. If somehow the hub came loose (dragging brake getting the hub smouldering hot?) then the key and nut would keep it on. I *think* (but could certainly be wrong) that putting grease or anti-seize on the taper would lessen that effectiveness of that interference fit, and make the whole set up much more reliant on the key and nut. I don't have much of a track record to say though, just an anecdotal evidence of my single old '63 PV (closing in on owning it for 20 years).

I also have the same experience with a little time taking an incredibly stubborn hub off. When I forst got my PV it had sat for 17 years, prior to that one of the rear brakes had stuck on and it was limped home. That overheated rear brake was incredibly difficult to remove. I soaked it with PBLaster. I got a huge puller and hammered it on tight. Heated the hub. Hammered it even tighter. Heated it. More PBLaster. Got a bigger hammer. Hammered it as hard as I physically could Nothing at all. Hammered it until I was about to rage out and put the hammer through the window or something, and finally decided I just needed to walk away from it and cool my head off some, red mist not being a particularly useful mechaniching skill. Several hours later, BANG, sounded just like a gunshot.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

My guess is the whole thing works as a system and you need a good keyway and key along with the right torque on the nut to hold it all together. I don't see need for alignment since drum orientation is not important I have used a light film of grease (applied and then wiped clean) for many years with no ill effects and drums always come off with a medium amount of effort. That has been on 122, p1800, and 544.

Tom;

Penetrating oils may help, but you MUST start out by using one of the approved pullers on this stretched joint, which applies concentric tension between lugs and axle...applying force non-concentrically or anywhere else (like edge of drum)...like so:

...will either not work, and/or break or damage things...once the proper puller is attached and putting the joint under mega-tension, some heat on the hub will help as it "grows" the hub a little, and maybe just enough to pop it apart...all this pain and suffering can be avoided by using the slightest film of graphite grease during last assembly...just saying...

See: http://sw-em.com/Brake_Drum_Notes.htm

Cheers

The puller I used attached to the lugs. The graphite grease is a good idea if I have to take the drums off again in the ner future. However, as in my case, most of the time the drums haven't been touched in 10 or 20 years and are a real bear to remove.

...near future or far future...graphite grease is absolutely the way to go...and I started using that (and recommending it) 30 years ago...

To reiterate what Ron said and add 2cents... The puller should pull on the drum studs while pushing against the center of the axle shaft. Any of the pullers that use slide hammers etc will just distort seals and backing plates etc...I have pulled lots of very stiff drums.....just time and tension