Ultimaker 2 machine definition variation for Cura 2.1.x that uses RepRap gcode flavor instead of UltiGCode. Useful in combination with OctoPrint

ultimaker2_reprap.json

{
    "id": "ultimaker2_reprap",
    "version": 1, 
    "name": "Ultimaker 2 RepRap",
    "manufacturer": "Ultimaker",
    "author": "Ultimaker",
    "icon": "icon_ultimaker2.png",
    "platform": "ultimaker2_platform.obj",
    "platform_texture": "Ultimaker2backplate.png",
    "file_formats": "text/x-gcode",
    "inherits": "ultimaker2.json",

    "machine_settings": {
        "machine_gcode_flavor": { "default": "RepRap" },
        "machine_start_gcode" : { "default": "G28 ; Home\nG1 Z15.0 F6000 ;move the platform down 15mm\n;Prime the extruder\nG92 E0\nG1 F200 E3\nG92 E0" },
        "machine_end_gcode" : { "default": "M104 S0\nM140 S0\n;Retract the filament\nG92 E1\nG1 E-1 F300\nG28 X0 Y0\nM84" },
        
        "material_print_temperature": { "enabled": "not (material_flow_dependent_temperature)" },
        "material_bed_temperature": { "enabled": "machine_heated_bed" },
        "material_diameter": { "enabled": "true" },
        "material_flow": { "enabled": "true" },
        "retraction_amount": { "enabled": "retraction_enable" },
        "retraction_speed": { "enabled": "retraction_enable" },
        "retraction_retract_speed": { "enabled": "retraction_enable" },
        "retraction_prime_speed": { "enabled": "retraction_enable" }
    }
}

You don't have to enable "use_materials" or make temperature serttings visible again or anything?

I tried doing this using only user-modifiable files (the .cfg files in the user's home, not the .json files inside the, usually cryptigraphically signed, .app MacOS application-package) but due to their limited syntax I could only get so far.
I guess I'll have to go your route to get the UM2extended to work with Octoprint.

Ultimaker/Cura#847 (comment)
aparently there is a secondary location outside the .app, so using JSON is now a viable option.

"use_materials" is new for Cura 2.2. Just setting the gcode flavor to anything not-UltiGcode is enough to make the material selection show up in 2.1. However I found out that properties such as printing temperature were being disabled by the UM2 definition.

I have updated the definition above to reenable these material properties. This should give you a printer that has all of the material-goodies of a non-UM2 printer.