7-bit circular buffer with bit option flag in a byte

Question

I'm hoping someone can help me on this.

I made a function for an 8051 microcontroller that accepts input from a button and I'm using a cyclic buffer of 8 bits to store key states so I can make debouncing not an issue.

The code to store data in the buffer and to check for one key is here:

VALIDPRESS equ 0Fh   ;0Fh = detect as valid if key held somewhat
CYCLICBUFFER equ 10h ;10h is a randomly picked value as example

mov R0,#CYCLICBUFFER ;memory location for key buffer
mov C,KEY            ;KEY = GPIO pin button is attached to
mov A,@R0            ;A = data found at address CYCLICBUFFER
rlc A                ;Shift in new detected value
mov @R0,A            ;Store updated byte to address CYCLICBUFFER
cjne A,#VALIDPRESS,nokey ;See if buffer contains bits in right order
;If it does, the key is valid
nokey:

There's only one thing... I have very limited memory available and I think I can somehow use one bit from the 8-bit buffer to store a flag. I want that flag to represent if that specific key is allowed to be held down or to be pressed only.

I'll show in the left column how data flows into my buffer as key presses are detected. What I'd like to have happen is whats shown in the right column below.

Let x equal unknown value, and a through m represent new button scan values 1 through 13 respectively, and let Z equal the custom flag that is never allowed to be changed by the keyscan routine.

Loop count,    Current data flow,    Desired data flow
    0          xxxxxxxx              Zxxxxxxx
    1          xxxxxxxa              Zxxxxxxa
    2          xxxxxxab              Zxxxxxab
    3          xxxxxabc              Zxxxxabc
    4          xxxxabcd              Zxxxabcd
    5          xxxabcde              Zxxabcde
    6          xxabcdef              Zxabcdef
    7          xabcdefg              Zabcdefg
    8          abcdefgh              Zbcdefgh
    9          bcdefghi              Zcdefghi
    10         cdefghij              Zdefghij
    11         defghijk              Zefghijk
    12         efghijkl              Zfghijkl
    13         fghijklm              Zghijklm

Is there an easy way to solve this without using lots of memory or lots of clock cycles?