OTA-program SPIFFS on the ESP8266
Basicly OTA supports the upload of *.bin files. Wether this contains compiled code or a filesystem (SPIFFS/LittleFS) is irrelevant. BUT The space for the OTA (partition) has to be min the size of the *.bin plus an overhead for processing. So with 4MB at all, you would need 2.5MB OTA for 1.5MB App/SPIFFS. To tweak partition sizes (or custom partitions) you have to go into the boards.txt file.OTA - Details and Info here 

File system - Details and Info here