A post blur of a sampled image is NOT the same as a pre-blur before sampling.
Sorry Grame I don't see your point. Imagine photons falling into bins. You can distribute them with a physical filter over a larger area. Then your photon counts will be exactly those you'd have without filter, convoluted by the mathematical filter (kernel) that matches your physical LPF (it may not be exactly Gaussian of course). If your physical LPF is not dispersive (which is to be hoped for) then it's the same filter for all RGB levels.
I don't see how applying a convolution on RAW data can be any different from having a physical LPF, (if we neglect the slight difference due to photons falling on photosite borders, of course).
I bet the differences you are reporting are due to blurring PROCESSED, sharpened images.
Yes there is natively a small artificial contrast induced by photosite borders, but if your raw converter allows applying a slight preblur before any other treatment, then you shouldn't see any aliasing at all. That's something fairly easy to implement in a raw converter, especially if users ask for it. So I don't see why having no physical LPF is a design flaw.
If you can show me evidence of the contrary, I'm ready to change my mind. :-)
Anyway applying low sharpening for Foveon images should solve your problem in practice. The only reason Sigma oversharpen them is to visually match 12 MP images. I don't mind slighty lower subjective resolution, especially for a compact camera. And the lower pixel count means I can put twice as many images on a memory card!