I'm afraid that I've not produced a 2.3.1 version of the wonderful documentation to go with it.

Please conduct your own beta testing; I have tested against the documented features and believe this works as described
but am not familiar with SPPC on 2.2 so I can't swear that I haven't made anything work differently.

The code changes applied are those in the documentation for SPPC 4.2.2 on osc 2.2
ANY SUBSEQUENT BUG FIXES TO CODE AMENDMENTS HAVEN'T BEEN INCLUDED, so that would be a good thing to cover
in your beta testing!

Package contains:
full set of amended and new files for SPPC 4.2.2 on 2.3.1 (no further add-ons)
full set of documentation for installing SPPC on 2.2 that you can follow for 2.3.1
full set of documentation for using SPPC on 2.2 that should be unchanged

Changes for 2.3.1:
(main places where the code amendments needed to be different, not just at a different line in the file)

catalog/admin/includes/boxes/customers.php - done differently on 2.3
catalog/admin/categories.php - no longer has a preview
catalog/admin/products_attributes.php - jquery buttons
catalog/admin/specials.php - fixed a bug preventing insertion of a special for a non-zero group with no group product price

catalog/includes/boxes/specials.php now catalog/includes/modules/boxes/bm_specials.php in a different format
catalog/includes/boxes/whats_new.php now catalog/includes/modules/boxes/bm_whats_new.php in a different format

catalog/includes/modules/new_products.php - quite different because of shop layout changes
catalog/includes/modules/product_listing.php - quite different because of shop layout changes
catalog/product_info.php - quite different because of shop layout changes
catalog/specials.php - quite different because of shop layout changes

More...