I've had partial success by doing this:

$ xkbcomp :0 default.xkb
$ vi default.xkb

comment out the two rules that cause syntax errors:

// interpret AccessX_Enable+AnyOfOrNone(all) {
// action= LockControls(controls=);
// };

// interpret MouseKeys_Accel_Enable+AnyOfOrNone(all) {
// action= LockControls(controls=);
// };

Now you can restore the settings back by running

$ xkbcmp default.xkb :0

if you screw up and end up with a nonfunctional keyboard.

Next

$ cp default.xkb mg.xkb
$ vi mg.xkb

key <RGHT> {
type= "PC_FN_LEVEL2",
symbols[Group1]= [ Right, Tab ]
};

$ xkbcomp mg.xkb :0

and now Fn+Right arrow is a Tab key.

Sadly I don't know enough about xkbcomp to see if it's possible to make it load incomplete maps on top of the current configuration, just to update a few symbols without replicating the full configuration.