I did contact work with the IRS from '06-'09, on the IMF and BMF (master file system) and I assure you there are no punch cards. Old school main frame assembly yes, but no punch cards.

Well, that's a bit of a relief.

The last I heard CADE2, the modernized (J2EE) system, had replaced the majority of the IMF functionality (IMF is the core of tax return processing). I'm not completely sure though as I haven't talked to those guys in over 4 years now.

Pun intended?

There should be a way to connect a card reader to a newish System Z machine - IBM is famous for its legacy support - likely via some wonky shit over SNA. MUMPS is also fairly portable to new architectures as it was cross platform out of the gate.

S/360 and descendant mainframes have gone through 3 generations of IO technologies: bus & tag channel (parallel copper cabling), ESCON (proprietary 1990s era fibre optic system), FICON (Fibre Channel based). Contemporary System z mainframes only support FICON, whereas card punches/readers were only ever bus & tag. However, I'm aware that some third party vendors offer protocol converters between bus&tag and ESCON, and between ESCON and FICON, (e.g. Optica Prizm) so in theory it is possible you could use a bus&tag-ESCON and ESCON-FICON converter in series to connect your legacy card punch/reader to a new System z mainframe. I don't know if anyone has tried it, so I don't know if it actually works. (I also don't know to what degree the IO code to handle physical card punches/readers still works in contemporary mainframe OSes, although given that virtual card punches/readers are still used under z/VM, it is possible that some of that code might still handle real ones.)

For no other reason than to satisfy my curiosity, I would love to see this done.

I was thinking of the support for RJE or some such might be less work - albeit likely much slower.