Given that Factorio 1.1.76 allows the loader entity to load trains, it seems like maybe this mod's implementation technique of having a bunch of hidden inserters is not necessarily the right one anymore. Any thoughts on this?

Not at all Miniloaders are insanely bad for UPS compared to normal loaders.
Someone did a benchmark while back and it's bad.

factorio.exe --benchmark-ticks 10000 --benchmark
windows 7, i5-3570

baseline (empty world, before placing the entities):
Performed 10000 updates in 471.805 ms
avg: 0.047 ms, min: 0.022 ms, max: 0.628 ms

baseline_chests (20k infinity chests):
Performed 10000 updates in 471.805 ms
avg: 0.047 ms, min: 0.022 ms, max: 0.628 ms

10k_inserters (10k max stacksize stack inserters, directly from chest to chest):
Performed 10000 updates in 21539.497 ms
avg: 2.154 ms, min: 0.860 ms, max: 5.406 ms

20k_inserters (20k max stacksize stack inserters, directly from chest to chest):
Performed 10000 updates in 45923.212 ms
avg: 4.592 ms, min: 1.790 ms, max: 18.806 ms

20k_loaders (10k x chest->loader->loader->chest):
Performed 10000 updates in 81389.618 ms
avg: 8.139 ms, min: 3.286 ms, max: 19.934 ms

20k_miniloaders (10k x chest->miniloader->miniloader->chest):
Performed 10000 updates in 160548.170 ms
avg: 16.055 ms, min: 11.889 ms, max: 82.721 ms

10k_10k_belt_inserters (10k x chest->inserter->4 belts->inserter->chest):
Performed 10000 updates in 77127.585 ms
avg: 7.713 ms, min: 5.665 ms, max: 14.863 ms

10k_10k_belt_loaders (10k x chest->loader->4 belts->loader->chest):
Performed 10000 updates in 82786.241 ms
avg: 8.279 ms, min: 3.684 ms, max: 18.379 ms

10k_10k_belt_miniloaders (10k x chest->miniloader->4 belts->miniloader->chest):
Performed 10000 updates in 175216.980 ms
avg: 17.522 ms, min: 12.789 ms, max: 42.115 ms

• Loaders do not provide negative filters
• Loaders cannot be connected to circuit or logistic network.

So, there's a reason for them to exist, but they should be used in very limited situations..

Can you convince Wube to implement those two features to loaders?