I know for sure that 86box does not accurately model the hardware behaviour. That's why I like FreddyV's PicoMEM. If the backfill doesn't work and I didn't blow up my SL, I can use it for other purposes.
86box does not have SL support (only for SL/2, which is probably incorrect, but I don't have...
I've not tried on real hardware, but I'm running 86box with a dump of my 1000 SL BIOS and it works with that. After adding 128 kB at the 512-640k window, the BIOS reports 'Memory Size = 768k' instead of 640k.
ms.com output with normal 640k:
ms.com output with added 128k (64k gained in DOS 3.3):
Your board seems like a nice solution for adding 128 kB to my Tandy 1000SL to get to full 640 kB that is not shared as video RAM. Adding memory to my 8086 system with an 8-bit ISA card will be slow, but for some programs slow is better than not running at all, so that's not an issue for me.
Scottmm78 posted a dump for BIOS ROM version 01.04.00 type 32 with English DeskMate 03.00.00.
My Tandy 1000 SL has Sharp ROMs with BIOS ROM version 01.04.01 type 40. I could upload those if somebody needs them.
This type isn't mentioned in the 'Tech Notes & Jumper Guide For The 1000's Vol. 1'...