As Belgian Mac's keyboard has never been correctly adapted, changes are bigger than the swapping keycodes as the canadian multitouch, we have to redefine @, #,€,`,[,],{,}. On Mac OS, to have {}, we use the CMD button with '(' and ')' , and for [ ], we use CMD + SHIFT + '(' and ')'. On linux, I've mapped it identically, but with ALT (left). So to write '{' we use ALT +'(' and for '[' we use ALT+SHIFT+'('.

juste replace in your /etc/X11/xkb/symbols/be file:

partial default alphanumeric_keys
xkb_symbols "basic" {
    include "latin"
    name[Group1]="Belgium";
    key <AE01>  { [ ampersand,          1,          bar,   exclamdown ] };
    key <AE02>  { [    eacute,          2,           at,    oneeighth ] };
    key <AE03>  { [  quotedbl,          3,   numbersign,     sterling ] };
    key <AE04>  { [apostrophe,          4,   onequarter,       dollar ] };
    key <AE05>  { [ parenleft,          5,    braceleft,  bracketleft ] };
    key <AE06>  { [   section,          6,  asciicircum,  fiveeighths ] };
    key <AE07>  { [    egrave,          7,    braceleft, seveneighths ] };
    key <AE08>  { [    exclam,          8,  bracketleft,    trademark ] };
    key <AE09>  { [  ccedilla,          9,    braceleft,    plusminus ] };
    key <AE10>  { [    agrave,          0,   braceright,       degree ] };
    key <AE11>  { [parenright,     degree,   braceright, bracketright ] };
    key <AE12>  { [     minus, underscore, dead_cedilla,  dead_ogonek ] };
    key <AD01>  { [         a,          A,           at,  Greek_OMEGA ] };
    key <AD02>  { [         z,          Z,      lstroke,      Lstroke ] };
    key <AD03>  { [         e,          E,     EuroSign,         cent ] };
    key <AD11>  { [dead_circumflex, dead_diaeresis,  bracketleft, dead_abovering ] };
    key <AD12>  { [    dollar,   asterisk, bracketright,  dead_macron ] };
    key <AC01>  { [         q,          Q,           ae,           AE ] };
    key <AC10>  { [         m,          M,   dead_acute, dead_doubleacute ] };
    key <AC11>  { [    ugrave,    percent,   dead_acute,   dead_caron ] };
    key <TLDE>  { [        at, numbersign,      notsign,      notsign ] };
    key <BKSL>  { [dead_grave,   sterling,   dead_grave,   dead_breve ] };
    key <AB01>  { [         w,          W, guillemotleft,        less ] };
    key <AB07>  { [     comma,   question, dead_cedilla,    masculine ] };
    key <AB08>  { [ semicolon,     period, horizconnector,   multiply ] };
    key <AB09>  { [     colon,      slash, periodcentered,   division ] };
    key <AB10>  { [     equal,       plus,   dead_tilde, dead_abovedot] };
    key <LSGT>  { [      less,    greater,    backslash,    backslash ] };
    include "level3(ralt_switch)"
};

and restart X to take effect