As promiced, my final solution.
I defined a variable:
ToolButtonHelpContext:integer;
Then, in TMainForm.OnAppIdle:
...
MPos := ScreenToClient(Mouse.CursorPos);
Control := ControlAtPos(MPos,[capfAllowDisabled,capfRecursive,capfAllowWinControls]);
if Control is TToolButton then
ToolButtonHelpContext := Control.HelpContext
else
ToolButtonHelpContext := 0;
....
In TMainForm, KeyPreview property is set to true, and in TMainForm.FormKeyDown:
...
VK_F1: if MainHelpContext <> 0 then
Application.HelpContext(ToolButtonHelpContext);