Using myst portals and train carts (as I do now) simply bypasses the problem in that the carts will stop immediately in place if the chunk they are moving to isn't loaded. They will then stay there until someone gets within range. My quarry on the other end is set up to stop operation if its buffer gets filled.
If everyone was to keep their spot in spawn loaded we would have all of spawn loaded all the time. Although I don't know, maybe this is already more or less the case due to some player typically being logged in there at any given time anyway?
Edit: Just came to think of a way to have teleport pipes that can handle the destination chunk not being loaded;
simply put a wooden pressure pad below the teleport pipe and feed the inverse signal to the engine that extracts items into the pipe.
As soon as an item is dropped from the pipe, it will land on the pressure pad and turn off the engine. Once the item despawns the engine will start again. You will still end up with items on the ground, but highly limited.
If you don't want to lose ANY items you can instead set an obsidian pipe below, which will suck up dropped items and transport them back into the output pipe. Then set a gate just after the obsidian pipe which will stop the engine while there are items in the pipe. The pump will pump our small loop full and then turn off until items start getting accepted into the teleport pipe again.
<-- E = Engine
E C = Chest (buffer)
TSIWCC W = Wooden pipe
S I = Iron pipe (output aimed towards teleport pipe)
OGS S = Stone pipe
--> T = Teleport pipe
O = Obsidian pipe (sucks up items dropped from teleport pipe)
G = Stone pipe with gate on it (turns off engine when items traverse)