Use for tin plate PS3 tender and boiler boards?

Sure it can, MTH no longer uses the relay board, the FET based board is used in all new steam.  Yes, you can load steam chain and sound files of your choice.  Note that there are some things you need to take note of, specifically if the boards are EMI boards or earlier non-EMI boards.  You need to make sure you have the proper version (or later) of the chain files for EMI boards.  The later versions should also work with non-EMI boards.   A cut from the Service Bulletin about the EMI boards and the chain files.

Software Compatibility:
You can mix and match EMI and non-EMI tender boards if you need to. However, there are minimum DSP (processor) and boiler code versions you MUST respect when using EMI boards. Below is the list:
DSP Code (Tender/O Diesel) – engine-3239.srec or higher
Boiler Code – boiler-3232.srec or higher

I'll have to see what the serial numbers are then.

How do I know the chain file min code version? Is there a certain date?

20-3499-2 | MTH ELECTRIC TRAINS (mthtrains.com)

Open the chain file ZIP (inside the overall ZIP with the sound file.  Here's an example PS/3 sound file for steam.

The boiler is rev 3366, and engine is rev 3409.  Obviously, both of these are later than the cutoff and are EMI board compatible.

Thank you!

I saw the 3226 for cv and got worried.

Here's your link, you're fine with the versions.