I think the ability to bring up an ingame menu would be really helpful for certain kinds of situations. I originally came up with the idea when I was deciding how to handle opening containers and possibly taking their contents, but I think there could be a variety of uses.
It would look something like this (rough mockup):

It could be drawn just using colored boxes and text, or defined with images (possibly some weird kind of HUD gauge)?
For dialogue trees, the top part could be done away with, since the other ship would simply send a message.
The SEXP would look like:
draw-menu
- variable to store choice in
- number of choice/response buttons
- button 1 text
- button 2 text
- etc
- [optional] top area text (top area not shown if this is not used)
I have no idea if this would be moderately difficult or nigh-impossible due to some aspect of the engine, so just tell me if it is and I'll forget about it.