I don't think the blue merc plugs will work in place of your brass plugs. The threads are way different.

I personally wish my boat had your brass plugs instead of the blue plastic ones, because the brass plugs don't break.

I broke a plastic one, and it would not back out at all. I took an old flat screwdriver, heated it up with a torch, melted a "slot" in the broken plug, cooled the screwdriver and turned it out like a screw.

And I broke one on the bottom of my fuel cooler and no sooner did I touch it with my 3/16" inch screw driver blade, it practically fell out. Not sure why you had to gorilla yours out. And my blue ones (and the one black one I have now) definatly don't leak.
Oh and here's what it all looks like.

Since they have that o-ring and are hollow inside, they are very easy to remove when they break. Without the o-ring, they have no clamping (is that the right term?) force.
Sorry you had trouble 2Suns.

As far as the threads being different, yeah, you are probably right. But Mercruiser uses a brass fitting for their knock sensor that looks a lot like that one in these photos, and the blue plug screws right into it.

The block drains on the houseboat engines are brass wing-nut "valves". I usually need a pair of pliers to loosen them, but not to tighten them, and no teflon tape needed. Won't break either.

I still like the hose drains on the babyliner, don't get much easier!