I don't know how would this work with a computer other than the P1/P2 models. As the P1 is a cut-down version of the P2 is easy to see how to proceed, but the P3/P4/P30/P40 models are improved versions and unfortunately I have no units of these models to test any of my designs. However, if this can solve issues for all the range of the computers I'll be happier than if it only solves issues for the P2.
I'm using both P2U and P30. My P2U has an single 64K Memory card, which also works with the P30 and vice versa. Therefor, if you do it right, your "Mammoth" should work together with P3, P30, P4, P40 too. Like helwie44 already wrote: All this Px machines starts in 48K mode.
I mean, it's not necessary to switch to a memory size of just 32 kB for a P1.
switch-configurable to 32, 48 and 64Kb
I actually had never found a P1 then and never did.
So the new card should behave exactly like the HARD-RESET or POWER ON on 4000h to FFFFh as 48 kB.
This will initially start a P1 (if available) and a P2 and work fine!
Also always starts a P2S, P2U, P3, P30 with the 48 kB memory.
Now it should be possible to make a BANKING via the PORT 78h.
Just like you will make it sure.
In order to make it easier for you, I agree with @Helwi44 that a 32K mode isn't necessary, because every P1 should also work with 48K and their owners will be very happy about the upgrade.
I'll follow your reports with great interest.