Well, in the meantime I have acquired not a 70 mm Sigma macro, but an Apo Lanthar 125 and 180, and I am now in the position to answer my own question:
Yes the short focus throw makes these lenses unsuitable for (my) infinity work with focus stacking. I have glued a cm/mm scale around the lens barrel to get repeatable focus settings. The 2 screen shots show 2 attempts of reproducing "12.98" mm on that scale, where 13.0 mm is infinity, according to the physical stop on the lens. Apo Lanthar 125, crops @100%.
And it gets worse.
Still looking for a good lens in the 70 mm range, I even dared to look at various varieties of the 75 mm Elmarit. As far as I can infer from images of these Elmarits, comparing their distance scale to that of my 85 mm Sonnar, I see that the Elmarit has a much shorter focus throw - and that's not a macro lens!
So, the focus throw is a feature to look for when buying a lens - one that seems to get little awareness.
If anybody knows of a good 70 mm lens with a looong focus throw, as the Zeiss lenses, I'm all ears.
Good light! - Hening.