Relic that CAUSES Imp status?

If you aren't using the Critical->Reflect effect, you could change that to Critical->Imp.  It's unusual and kinda goes for what you're seeking.

Code:

C2/0A86: A9 24        LDA #$24  (Reflect spell ID)

Change to  A9 23 so that it casts Imp, provided you haven't moved the Imp spell

Note that if the character has protection from Imp status (from another piece of gear) the spell will likely miss (I haven't tested that).  If it doesn't, then the imping will be permanent because the protection is there.  You may wish to play with this a bit.

EDIT: Actually, even if you are using Wall on Low HP, that equipment byte has two unused bits.
Here's some code I used to add extra relic functions, tweaked to your situation.

Code:

C2/0A82:            [Replace original code for Wall on Low HP]
    22 8C FF C0                        JSL $C0FF8C        [Subroutine checks extra equipment bits]
    60                                RTS
    EA EA EA EA EA EA EA EA EA        NOP out remaining original code
    
C2/64CC:            [Some original code for Wall on Low HP, in a subroutine to set up an auto spell.  Must be in C2 bank]
    85 B8                            LDA $B8
    A9 26                            LDA #$26
    20 91 4E                        JSR $4E91
    6B                                RTL

C0/FF8C:            [Checks extra bits on the equipment, sets spells]
    4A                LSR            [Check third bit (Wall on Low HP in FF3USME)]
    90 08            BCC
    48                PHA
    A9 EE            LDA #$24    [Load Reflect]
    22 CC 64 C2        JSL setUpSpell                ***If you move C2/64CC code somewhere else, update this line***
    68                PLA
    4A 4A 4A        LSR x3        [Check sixth bit (First ? after Double Gold Pts in FF3USME)]
    90 08            BCC
    48
    A9 1D            LDA #$1F    [Load Haste]
    22 CC 64 C2        JSL setUpSpell                ***If you move C2/64CC code somewhere else, update this line***
    68                PLA
    4A                LSR            [Check seventh bit (Last ? after Double Gold Pts in FF3USME)]
    90 08            BCC
    48                PHA
    A9 2C            LDA #$23    [Load Imp]
    22 CC 64 C2        JSL setUpSpell                ***If you move C2/64CC code somewhere else, update this line***
    68                PLA
    6B                RTL

Give it a try if you like.