Well, sorry this took forever, but the code used in JUMP 1/2 is public, so I just pulled it from their resource pack. Don't credit me for this, nor will I be submitting it, but this fulfills the request as described.
Codemain:
LDA $9D ;\ If sprites locked,
ORA $13D4|!addr
BNE .return ;/ branch
LDA $60
BEQ +
REP #$20
INC $96
SEP #$20
LDA #$01
STA $0F00|!addr
BRA ++
+
LDA $0F00|!addr
BEQ ++
STZ $0F00|!addr
LDA #$10
STA $7D
REP #$20
INC $96
SEP #$20
++
STZ $1412|!addr ; Disable vertical scroll
DEC $1468|!addr ; Scroll layer 2 downwards
LDA $1468|!addr ;\ If not time for layer 2 to return,
BNE .return ;/ branch
LDA #$C0 ;\ Reset layer 2 position
STA $1468|!addr ;/
.return
RTL
Edit: forgot the .return label.
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