Circling back to this to close off the thread with an answer at least.
Here are the changes I used to convert the bootloader codebase to build for 168p.
Note that this change set is not complete enough to fully handle choosing between either MCU - some changes will become hard coded for 168 instead of 328 so it would not be suitable as a formal submission as it is. Unfortunately I got busy and now also have plenty of 328s back in my inventory. Also I cannot guarantee it's 100% correct but it seems to work well for me on the ~8 nodes I've used it on:
--- MySensorsBootloaderRF24/boards.txt  2020-10-14 22:50:32.182131357 +1030
+++ MySensorsBootloaderRF24.at168clean/boards.txt       2020-10-14 21:34:35.482909793 +1030
@@ -1,18 +1,18 @@
 ## Add these lines to your boards.txt file in the Arduino installation directory
 ##
-## Arduino Pro or Pro Mini (3V3 & 5V, 1 - 16 MHz) w/ ATmega328 MYSBootloader
+## Arduino Pro or Pro Mini (3V3 & 5V, 1 - 16 MHz) w/ ATmega168 MYSBootloader
 ## -------------------------------------------------
 
 menu.frequency=Frequency
 
-MYSBL.name=ATmega328 with MYSBootloader 1.3.0
+MYSBL.name=ATmega168 with MYSBootloader 1.3.0
 
 MYSBL.upload.tool=avrdude
 MYSBL.upload.protocol=arduino
 MYSBL.upload.maximum_size=30720
 MYSBL.upload.maximum_data_size=2048
 MYSBL.bootloader.tool=avrdude
-MYSBL.build.mcu=atmega328p
+MYSBL.build.mcu=atmega168p
 MYSBL.build.board=AVR_UNO
 MYSBL.build.core=arduino
 MYSBL.build.variant=standard
diff -u MySensorsBootloaderRF24/Core.h MySensorsBootloaderRF24.at168clean/Core.h
--- MySensorsBootloaderRF24/Core.h      2020-10-14 22:50:32.181131338 +1030
+++ MySensorsBootloaderRF24.at168clean/Core.h   2020-10-14 22:52:38.760633493 +1030
@@ -14,8 +14,8 @@
 #define MYSBOOTLOADER_VERSION ((MYSBOOTLOADER_MINVER << 8) + MYSBOOTLOADER_MAJVER)
 
 // size setting ********************************************************************************************************
-#define BOOTLOADER_SIZE        (2048)
-#define BOOTLOADER_START_ADDRESS (0x8000 - BOOTLOADER_SIZE)
+#define BOOTLOADER_SIZE        (1024)
+#define BOOTLOADER_START_ADDRESS (0x4000 - BOOTLOADER_SIZE)
 
 // DEBUG led patterns  *************************************************************************************************
 
diff -u MySensorsBootloaderRF24/Definitions.h MySensorsBootloaderRF24.at168clean/Definitions.h
--- MySensorsBootloaderRF24/Definitions.h       2020-10-14 22:50:32.181131338 +1030
+++ MySensorsBootloaderRF24.at168clean/Definitions.h    2020-10-04 23:30:41.494466259 +1030
@@ -9,7 +9,7 @@
 #define Definitions_H
 
 #define FIRMWARE_BLOCK_SIZE    (16)
-#define EEPROM_SIZE 1024                       // 1024 bytes for ATMEGA328
+#define EEPROM_SIZE 512                        // 512 bytes for ATMEGA168
 
 #define false  (0)
 #define true   (1)
diff -u MySensorsBootloaderRF24/Makefile MySensorsBootloaderRF24.at168clean/Makefile
--- MySensorsBootloaderRF24/Makefile    2020-10-14 22:50:32.181131338 +1030
+++ MySensorsBootloaderRF24.at168clean/Makefile 2020-10-14 21:43:48.433871688 +1030
@@ -1,6 +1,6 @@
 PROJECT = MYSBootloader
 
-MCU = atmega328p
+MCU = atmega168pa
 CLK = 1000000L
 BAUDRATE = 9600
 
@@ -8,16 +8,16 @@
 ISP_PORT = com5
 ISP_SPEED = $(BAUDRATE)
 ISP_PROTOCOL = stk500v2
-ISP_MCU = m328p
-ISP_HFUSE = DA
-ISP_LFUSE = F7
-ISP_EFUSE = 06
+ISP_MCU = m168p
+ISP_HFUSE = DD
+ISP_LFUSE = FF
+ISP_EFUSE = F8
 ISP_ARGS = -c$(ISP_PROTOCOL) -P$(ISP_PORT) -b$(ISP_SPEED) -p$(ISP_MCU)
 
 BINPATH = /usr/bin/
 
 CFLAGS = -funsigned-char -funsigned-bitfields -DF_CPU=$(CLK) -DBAUD_RATE=$(BAUDRATE) -Os -ffunction-sections -fdata-sections -fpack-struct -fshort-enums -mrelax -Wall -Wextra -Wundef -pedantic -mmcu=$(MCU) -c -std=gnu99 -MD -MP -MF "$(@:%.o=%.d)" -MT"$(@:%.o=%.d)" -MT"$(@:%.o=%.o)" 
-LDFLAGS = -nostartfiles -Wl,-s -Wl,-static -Wl,-Map="$(OutputFileName).map" -Wl,--start-group -Wl,--end-group -Wl,--gc-sections -mrelax -Wl,-section-start=.text=0x7800 -mmcu=$(MCU)  
+LDFLAGS = -nostartfiles -Wl,-s -Wl,-static -Wl,-Map="$(OutputFileName).map" -Wl,--start-group -Wl,--end-group -Wl,--gc-sections -mrelax -Wl,-section-start=.text=0x3800 -mmcu=$(MCU)  
 
 
 all: clean out
Hopefully this helps someone...
Cheers
Charles