Also I like the idea of conveyor belts actually interacting with the workshop directly as opposed just moving stuff from one stockpile to another, though maybe if stuff just got moved from one workshop to another, going from one bin in a workshop to the other, it might save the potential lag chaos (not sure, but it might be possible) you could get with stuff going round in a loop indefinitely with possible quantum stacking, also it'd be curious if you could combine a pressure plate with all this and so when a bin in a workshop is full presumably being inactive, it can send a signal to somewhere on the conveyor belt and 'turn' the direction of traffic.
I'm more of the "simple conveyor" type of person (ok, so maybe with a 'workstation' and a dedicated worker, you could arrange redirection of an item, but I'd prefer Dwarf technology to be basically A->B), but what you say about bins actually strikes a chord for how a 'realistic' system could be arranged.
Conveyor units, built as per axles in arbitrary-long horizontal and vertical increments (1..10, perhaps requiring a mix of something like logs, leather and mechanisms each in an appropriately scaled proportion) will take anything put onto them (at any point throughout their length) and convey them at a certain speed until they fall off their end.
But, they need bins (or equivalent, i.e. hoppers) at
all entry points and exit points. For manual entry, the bin will accept items as per current bins and will release each item one at a time (de-stacking if necessary, noting that by this time re-stacking of compatible items will almost certainly have been sorted out) onto the conveyor if currently
empty. Manual insertion points have to be assigned to accept particular good types/qualities/etc, stock-pile-wise.
I'm toying with junctions, though. I normally have no problem with conveyor dumping onto conveyor, unregulated, but you could end up with a build-up that way, so maybe you also need a bin system buffering every entry-point. This would apply for both mid-belt insertions and entry at the beginning (after all, you could get three feed-belts trying to dump material onto a trunk one, and you really shouldn't leave that exploit open).
And make sure the bins can 'fill', and if the bin is full the feed (or feeds) to it and any manual tasks to put more goods into it are frozen. Stop the conveyors, and abort (or make unallocatable) the bin-filling haulage jobs while this state of affairs exists (could mean a lot of start-stop of process, job cancellations, etc, and the repercussions of power-transfer system along a halted length of belt).
And should the exit be equipped by a built bin (with similar fillable quality stopping the feed) as part of the retrieval system? Otherwise, you'r quantumly storing on the output, even if you stopped the possibility of quantumly storing on the belt itself.
An alternative is to implement like axles and gears, with bins as full-on junctions, but you really need it all to add up to have a conveyor coming from beneath a bin/hopper.
As for capacity, rather than based on a single-item per tile, perhaps go by weight/other measurement of size? Thus raw stones might actually 'reserve' two or three belt lengths by dint of their weight (and, more bin/hopper space), whereas many wooden earrings could occupy a single belt tile. Those are fine-tuning details, of course, and this just a speculative idea.