These all use the same internal filters for SSB/CW.

If I remember correctly, the 817 has one filter slot and the others have 2 filter slots.

Anyway the question is, are these filters in the IF stage?

I think you can get blank PC boards made to fit the filter slots to build your own filters using cannibalized crystals since Collins no longer makes them.

But here's the question.  Would it be feasible to take one of these boards and just make a pass-through with no filter, so that when the filter option is selected in the radio menu, you still get a signal?

I'd like to build a board to install an SDR dongle inside the radio and have a USB port on the back to make a panadapter.  Could I use the blank filter board as an IF tap location and as mechanical support for the SDR dongle?  I think this would be an ideal use for an unused feature in the radio in the absence of filter availability.

And, the audio filters in many SDR software apps make up for the loss of available IF stage filters in the radio.

It's been done several times.

https://ve3ips.wordpress.com/2016/08/12/panadapter-yaesu-ft-817-897-857-rtl2832u-out-if-mf-6833-mhz/

http://www.w1ghz.org/PCBproj/Low-cost_Panadapter_for_FT-817.pdf

I just was curious if the unfiltered IF was available at the slot for the narrow SSB and CW filters, but I guess that is a later stage of the receiver.

Wish I could find one of those for a filter for the FT-847. The Collins filter for the 857/897 is reasonably available, but the 847 ones are different and unobtanium.

I started down the panadapter some time ago and never finished the project. IIRC I needed a 455 kHz filter to finish the board. This is where I started, it's evolved https://www.qsl.net/rf-seismograph/