I'm a little stumped on this one so far. Just to make sure I have the right idea, you have something like this, where '|' is where the cursor comes up:
someFunc (x|);
'x' is a pointer to a structure. You type '->' and then you get the completion dialog for the structure members, and when you select one, the space gets removed between 'someFunc' and the '('? Are you selecting the structure member with the mouse, or a set of keypresses, and if so what keypresses? And what's your emulation?