I looked at building a crossfeed add-on board for one of my amp designs a year or two ago. Looking into it through there were several software crossfeed plug-ins for popular audio players that seemed to be highly rated, so I never bothered. At least one of those was for-pay commercial software too, as I recall, and rather pricey.
Software doesn't help much though if the source is a turntable or or non-PC gizmo!
Dr Meier did that original post on Headwize way back when (the excerpts in your second link in post #2). Since then it looks like he formed Meier audio, a commerical company, if I have the history right. Here is his crossfeed page:
Meier Audio
EUVL has a good point. Crossfeed may be best done on the source end of things (that is where I was going to put it) so a low Z output headphone amp can drive the headphones directly.