Plastics are really bad mate, they are strong but have shocking Coefficient of thermal expansion you will litterally see it moving!
If you sealed the tube up some how and drew the air though it then as long as it doesn't actually cut the mirror you should be right. but really the difference between 3 and 4 parts to the truss will be about 3 minutes of set up. Ally and Carbon fibre tubes are light so it wont take much wieght
As for nodal points you make a common bracket give the joints a close tolerence eg use a set of verniers to measure the bolt then find a corrosponding drill bit to clear it just you wont have any issues I do this for some of the things i build when i need the tolorence. I am a ATMer like yourself
its just i have very little time presently to get into it!
Brendan