I have a gut feel aversion to telescoping tongues as it's so easy to make one that isn't structurally sound. I think all this use of receivers (where both the receiver and the stinger are thick wall but short tubes) gives people bad ideas!
So if you're going to have a telescoping tongue, I would like to see:
- a long, long 'receiver' section - like a couple of feet long minimum;
- no holes in the tongue at the front end of the 'receiver' - so any holes for pins well behind the front end (yes, that will make it awkward to put the pins in and out and, yes, you will have to lie in the dirt to do so);
The problem with pin holes in a telescoping tongue is that you may want some extra holes in the tongue for the 'retracted' position. No problem, as long as these are in the front half of the exposed tongue length where the tongue strength isn't too critical.
The receiver will need to be supported at least as well as a single tongue would be - so for example, welding it just to the front cross-member won't be sufficient.
Here's a sketch:

- telescopic-tongue.JPG (26.53 KiB) Viewed 1558 times