i came to the conclusion that the core directive just didn't support what i was trying to do so i ended up making my own ;)

it's 99.9% the same as the one in the core but allows you to pass an argument variable.

this is the view - i've named it umb-button-group-extended.html:

<div class="btn-group umb-button-group" ng-class="{'dropup': direction === 'up'}">

    <umb-button ng-if="defaultButton"
                alias="{{defaultButton.alias ? defaultButton.alias : 'groupPrimary' }}"
                type="button"
                action="defaultButton.handler(args)"
                button-style="{{buttonStyle}}"
                state="state"
                label="{{defaultButton.labelKey}}"
                label-key="{{defaultButton.labelKey}}"
                shortcut="{{defaultButton.hotKey}}"
                shortcut-when-hidden="{{defaultButton.hotKeyWhenHidden}}"
                size="{{size}}"
                icon="{{icon}}"
                add-ellipsis={{defaultButton.addEllipsis}}>
    </umb-button>

    <button data-element="button-group-toggle"
            type="button"
            prevent-default
            class="btn btn-{{buttonStyle}} dropdown-toggle umb-button-group__toggle umb-button--{{size}}"
            ng-if="subButtons.length > 0"
            ng-click="toggleDropdown()"
            aria-haspopup="true"
            aria-expanded="{{dropdown.isOpen}}">
        <span class="caret">
            <span class="sr-only">
                <localize key="{{labelKey}}">{{label}}</localize>
            </span>
        </span>
    </button>

    <umb-dropdown ng-show="subButtons.length > 0 && dropdown.isOpen"
                  class="umb-button-group__sub-buttons extended"
                  on-close="closeDropdown()"
                  deep-blur="closeDropdown()"
                  ng-class="{'-align-right': float === 'right'}">
        <umb-dropdown-item ng-repeat="subButton in subButtons">
            <button data-element="{{subButton.alias ? 'button-' + subButton.alias : 'button-group-secondary-' + $index }}"
                    type="button" ng-click="executeMenuItem(subButton)"
                    hotkey="{{subButton.hotKey}}"
                    hotkey-when-hidden="{{subButton.hotKeyWhenHidden}}"
                    prevent-default>
                <localize key="{{subButton.labelKey}}">{{subButton.labelKey}}</localize>
                <span ng-if="subButton.addEllipsis === 'true'">...</span>
            </button>
        </umb-dropdown-item>
    </umb-dropdown>

</div>

and this is the js:

(function () {
    'use strict';

    function buttonGroupExtendedDirective() {

        function link(scope) {

            scope.dropdown = {
                isOpen: false
            };

            scope.toggleDropdown = function () {
                scope.dropdown.isOpen = !scope.dropdown.isOpen;
            };

            scope.closeDropdown = function () {
                scope.dropdown.isOpen = false;
            };

            scope.executeMenuItem = function (subButton) {
                subButton.handler(scope.args);
                scope.closeDropdown();
            };

        }

        var directive = {
            restrict: 'E',
            replace: true,
            templateUrl: '/App_Plugins/UmbLogExporter/backoffice/umbLogExporter/directives/umb-button-group-extended.html',
            scope: {
                defaultButton: "=",
                subButtons: "=",
                state: "=?",
                direction: "@?",
                float: "@?",
                buttonStyle: "@?",
                size: "@?",
                icon: "@?",
                label: "@?",
                labelKey: "@?",
                args: "@?"
            },
            link: link
        };

        return directive;
    }

    angular.module('umbraco.directives').directive('umbButtonGroupExtended', buttonGroupExtendedDirective);

})();

it can be used in the views as:

<umb-button-group-extended
    ng-if="vm.buttonGroup"
    default-button="vm.buttonGroup.defaultButton"
    sub-buttons="vm.buttonGroup.subButtons"
    args="{{myObjectOrProperty}}"
    direction="down"
    float="right">
</umb-button-group-extended>

and wired up with the following js:

vm.buttonGroup = {
    defaultButton: {
        labelKey: "general_view",
        hotKey: "",
        hotKeyWhenHidden: true,
        handler: function (myObjectOrProperty) {
            //do something here
        }
    },
    subButtons: [
        {
            labelKey: "actions_download",
            hotKey: "",
            hotKeyWhenHidden: true,
            handler: function (myObjectOrProperty) {
                //do something here
            }
        }
    ]
};

passing an object or property to the args property on the scope will pass it up to the handlers for the default and sub buttons.

hope someone out there finds this useful ;)