This code makes use of SendMsg to do extra IPC calls. It uses IPC_ISMAINWNDVISIBLE=900 to check if the main window is visible. It also sends a WM_COMMAND=273 to select the main window toggle command.

WM_COMMAND=273
WM_WA_IPC=1024
IPC_ISMAINWNDVISIBLE=900
WINAMP_MAIN_WINDOW=40258
 
'Show main window
if (SendMsg(WM_WA_IPC, 0, IPC_ISMAINWNDVISIBLE) = 0) then
 SendMsg WM_COMMAND, WINAMP_MAIN_WINDOW, 0
end if
 
'Hide main window
if (SendMsg(WM_WA_IPC, 0, IPC_ISMAINWNDVISIBLE)) then
 SendMsg WM_COMMAND, WINAMP_MAIN_WINDOW, 0
end if