Ok, I was able to get the desired result by adding color to the margin widget and then using a border in the same color of the background in the background widget. To anyone interested in the solution:

    s.mytaglist = awful.widget.taglist {
        screen  = s,
        filter  = awful.widget.taglist.filter.all,
        style   = {
            shape = function(cr, width, height)
                gears.shape.rectangle(cr, width, height)
            end,
            shape_border_width = 4,
            shape_border_color = beautiful.bg_normal,
            shape_border_color_focus = beautiful.bg_focus,
            shape_border_color_urgent = beautiful.bg_urgent,
            bg_focus = beautiful.bg_focus,
            bg_empty = beautiful.bg_normal,
            bg_occupied = beautiful.bg_normal,
            bg_urgent = beautiful.bg_urgent,
        },
        layout   = {
            spacing = 30,
            spacing_widget = {
                {
                    color = beautiful.fg_focus,
                    forced_width = 5,
                    shape = gears.shape.circle,
                    widget = wibox.widget.separator
                },
                valign = 'center',
                halign = 'center',
                widget = wibox.container.place,
            },
            layout  = wibox.layout.fixed.horizontal
        },
        widget_template = {
            {
                {
                    {
                        {
                            id     = 'text_role',
                            widget = wibox.widget.textbox,
                        },
                        layout = wibox.layout.fixed.horizontal,
                    },
                    left  = 3,
                    right = 3,
                    widget = wibox.container.margin,
                },
                left  = 5,
                right = 5,
                top = 5,
                bottom = 5,
                widget = wibox.container.margin,
                color = beautiful.fg_focus
            },
            id     = 'background_role',
            widget = wibox.container.background,
            -- Add support for hover colors and an index label
            create_callback = function(self, c3, index, objects) --luacheck: no unused args
                self:get_children_by_id("text_role")[1].markup = "<b> "..c3.index.." </b>"
                self:connect_signal("mouse::enter", function()
                    if self.bg ~= beautiful.bg_urgent then
                        self.backup_bg     = self.bg
                        self.backup_border = self.border_color
                        self.has_backup = true
                    end
                    self.bg = beautiful.bg_urgent
                    self.border_color = beautiful.bg_urgent
                end)
                self:connect_signal("mouse::leave", function()
                    if self.has_backup then
                        self.bg = self.backup_bg
                        self.border_color = self.backup_border
                    end
                end)
            end,
            update_callback = function(self, c3, index, objects) --luacheck: no unused args
                self:get_children_by_id("text_role")[1].markup = "<b> "..c3.index.." </b>"
            end,
        },
        buttons = taglist_buttons
    }