Not without breaking ToA via a third-party key-intercept map that would turn Shift-Down and Shift-release into separate keys, afaik. Doing so would allow Shift to trigger a macro (use a hardware/mouse macro corresponding to the single, rather than ingame, for reliably instant effectiveness) which displays all your Shift keys and activates Shift key-state. Shift-release would correspond to another hardware macro that would close the hotbar.