I haven't had valve covers blown off before, but this scenario would be possible: No breather cap installed => your crankcase vapors stay in there, just a little bit gets sucked away by the PCV => crankcase full of blow-by vapors, which are often enriched with fuel => vapors ignite with an ever so small spark (steel rockers hitting top of valve stem?)

That is possible, quite apart from the fact, that for example a cylinder which is dead for a short time will really press unburnt mixture in the correct ratio for burning past the rings. So if your engine cycles a couple of times without firing you WILL have the correct mixture for explosions in your crankcase (this is not a "may have" issue!). Rings not sealing properly (you said it uses a bit of oil) will accelerate this fact.

A leaking carb will cause gas to be present on the base of the intake manifold. This will be drawn into the cylinders on the first couple of turns wich won't fire because the mixture is much too rich. But this enrichend mixture gets past your rings even more easily. Rich mixture in cold starts washes down the cylinder walls and causes more friction and more oil to be used.

As you see, there are very many possibilities as to what might be causing this. But blowing off valve covers is certainly something I would look into more deeply: For all you know there might be gas sitting somewhere in your engine in large quantities and could actually cause a considerable fire.

I would certainly install a breather on one valve cover and the PCV on the other, breather on the driver side, PCV on the passenger side (that's because of the rotation of the crankshaft in the crankcase: it acts as a pump and pulls down on the gases on the driver side and pushes them up on the passenger side).
I would also check the carb for leaks internally, maybe just get a gasket set and rebuild it, done in half a day.
I would take off the valve covers and check for faults in the castings. Sometimes valve guides crack without hurting the heads themselves, in which case a spark which goes off too soon or too late can cause the igniting mixture to creep up a worn or cracked valveguide.
I would double recheck my timing all along the power curve und also recheck my valve setting, it's very easy to get one of them wrong and to be holding one valve open just ever so slightly, which would force mixture up into the bowls, from there it can get into the valve covers by way of cracks easily.

As you see, many "I woulds", but what you do is entirely up to you of course. I cannot promise that this hasn't been happening in small bits along the way, when driving you might not notice a small leak somewhere, but your engine parts may. This can certainly hurt other parts, too.

Hope it's of any help,
Max

A small note: gas won't stay in the oil for long, as soon as the oil is warm it is hotter than the boiling point of gasoline, so the gasoline will evaporate out of it and leave clean oil.
And: a faulty spark wire might cause a spark to go directly to the valve cover, which is insulated from the cylinder head by an eightth inch of gasket. The spark can jump that gap inside the valve cover. Combine that with the mixture in your crankcase due to the above mentioned and you're done (The more I think about it, the more likely this version seems to me... Check your wiring)