Is it feasible to add a Settings option to disable the threshold limits? For some uses, it would be good to see everything being requested and provided even if it's too little for a train to be sent.

Not possible.
LTN doesn't even store anything below thresholds and these Readers only show what LTN knows.

I thought that might be the case.

What would be a good way to report a "provide" amount but still prevent a train from coming to pick it up? I tried disabling the stop but that also disables the amount. I tried changing the max train length so that there were no matching trains, which works, but that seems like a poor hack.

I'm trying to coordinate between multiple construction providers and multiple construction sites. Wires don't work because of the lag (train trip) between subtraction from the provider and addition to the requester. This mod handles that problem but I still want to limit when a train can come so there is time to accumulate as much material as is practical.

"coordinate between multiple construction providers" sounds to me like overcomplicating things.
LTN will handle multiple providers and requesters by itself without complex circuitry.

If you want to really overengineer things:
Wires could work if providers constantly update memory cells with train inventory while a train is parked and requesters resets the cell when a train leaves.
Indexing memory cells with train id would allow keeping track of multiple deliveries.

double post from mobile

The LTN works great. It's getting the correct building materials to the provider station's buffer chests that's the trouble.
https://factorioprints.com/view/-MNZWdWosuqr3vtaC2hD

The solution I settled on was to use the Network ID. The construction network is 4096 and all construction sites set that ID. I configured the combinators to monitor 4096+2048=6144. The providers have a constant network ID of 2048 and they add that again when it has determined it has sufficient inventory for a train to be called, thus matching the requesters and completing the circuit.

Now each provider can see what is being requested and the total being readied at all providers yet only enable the train when it wants. This prevents trains coming to take away only a few items (because my thresholds are just 1).

I have some clean up to do with non-construction sites that didn't set a network ID but I can mostly ignore those with some additional constants to cancel-out those specific requests. Only brick is a problem since it's the only construction item that is also requested in bulk.