I have read that there shouldn't be any supply registers on the end of a duct. Not sure about on the side, near the end.
I don't know about a duct off the plenum at the end of the plenum. Seems like you would want it there. On a main duct trunk, you don't want one at the end, because most of the air will go out that one and not out the ones on the sides. With the plenum, I would want the air to be able to get out to the ducts as easy as possible.

The system I just had installed, has the main supply duct trunk off the side of the plenum (at the end) Also at that end of the plenum, is a register into a bedroom. Off another side of the plenum, is a register into another bedroom.
The house has no attic or basement and this was the only way to do it. I needed to block most of the air to these registers, and they still blow a lot of air.
I may add another register near the end of the main trunk (only 14' long) and restrict airflow more to the 2 closest registers.

I'm not a "pro" but then again the "pro" I hired to install my system, did so many things wrong, not to code, dangerous (elec) that I kicked him out and repaired his screw ups, then hired a company to repair freon tubing and charge it up.
A nightmare, but now it's "cool"
Good luck

over40,
Thanks for responding. I'm probably confusing my terminology, by "extended plenum" I mean the main supply run that runs the length of the house (in the basement), where the ducts are tapped off of. I think that is what you are referring to as the duct trunk. So, I have moved a duct "take-off" from closer to the middle of the duct trunk to near the end (about 12" from the end), but still on top of the duct trunk. Because you're supposed to have 18-24" of dead space at the end of the duct trunk, I am thinking of lengthening the duct trunk another 12".

My concern was whether it was okay to do that, and whether there are any "gotchas" to be aware of.

I would not bother extending the trunk.
My house has no attic, and the trunk had to go inside a soffit thru 1 room. At the end of the 11' room the trunk makes a 90 degree left turn, and goes another 5'. 6" from the end of the trunk, a register comes off it, thru a wall to the living room. At the end of the trunk, before it makes the 90 turn to the left, a register comes off it on the right, thru a wall to the kitchen.
In the middle of 11' trunk, there is a register in the middle for the room the trunk is in.

Then! there is a register directly off the plenum, into a bedroom. And off another side of the plenum, is a 3' long trunk, with a register in the end. Just before the end of this short trunk, it makes a 90 degree turn, and attaches to the 11' trunk.

Lots of no no's ! But it works. The 1 bedroom with the register off the plenum, is the coldest, even with 1/2 of the register box blocked off. That BR is a guest room and we leave the door open when not occupied.