i don't know how to do it with the Styles and stuff used for scripts but i think i remember how to do it with API.
heres the code in the best way i show you (sorry i havent made a script in ages dunno the calls to use the API)
code:
var GWL_EXSTYLE = -16
var WS_EX_TRANSPARENT = 0x20
var temp
temp = GetWindowLong(CtrlWnd, GWL_STYLE)
SetWindowLong(CtrlWnd, GWL_EXSTYLE, temp | WS_EX_TRANSPARENT)
(remember that CtrlWnd contains the handle of the RichTextBox not the entire window, im pretty sure your given the handle of the control when you create it (rtb = MsgPlus.CreateWnd(... or whatever it is))
hopefully that can solve your problem.