Until a few years ago every engine block of every family car was made out of cast iron ... and they were rebuilt which involved cleaning. I do not see why this block is any different and why there is such anxiousness.
Find somebody who rebuilds Chev/Ford v8's before they became alloy blocked
The reason that this block is different from classic US cast iron blocks, is that the ends of all of the drilled oil galleries are plugged with hammered-in aluminum plugs. This was not the technique for most cast iron blocks. Most used expandable "freeze plugs" made of steel, or threaded steel inserts. These are tolerant of higher concentrations of caustic.
As Richard and Gordon note, a quick flash of diluted caustic cleaning solutions is not likely to do any lingering damage to the old cast iron Alfa engines. Also as Gordon notes, soaking one for a long time in a hot, high-concentration caustic solution is likely to result in the need to replace the aluminum plugs. I'm not sure what it would do to the brass/bronze (?) intermediary gear bushing in the front and back of the timing chain cabinet.
This brings us to "what to do about cleaning the insides of the oil galleries?
If I recall correctly, one end is open, and you can use a gun-rod cleaner. Use mineral spirits, and scrub.
The hot spray with low-concentration of caustic is probably OK, but again, the surfaces of the bushings should be considered.
I am generally content with hand-cleaning. This involves lots of plastic-bristle brushes, mineral spirits, MEK, rags, and compressed air.
The argument for a clean engine is sound. However, there is a point at which we must wonder what is enough. Remember that the old air-cooled VW engines did not have a filter. They used a fairly coarse screen, and a centrifugal slinger that separated out the bigger chunks. Of course, we want to preserve our valuable and often rare Alfa engines, so clean is good. Re-drilling the oil galleries strikes me as perhaps more than a bridge too far.