sure you can use = or + via lua.
function keyboard(eventType, character, keycode, modifiers)
if eventType == eEvtKeyUp then -- key released
if character == "r" then ... end
elseif eEvtKeyTextInput then -- special keys (down/hold)
if character == "+" then ... end
if character == "-" then ... end
... the only problem was that + key only works on keydown/hold & not key released for some reason. - works for either. keytextinput & keydown will continuously trigger every loop/frame until you release the key. I prefer to always use key released where possible. Also I'm not sure if arrow keys can be accessed via lua or not but you can access more keys than you can via the editor key actions.
Here is a list of key events, codes & modifiers that David provided me with...
// key events
// key modifiers
// key codes