I just noticed passing through horizontally placed item frame (feature from Quark) aren't supported right now. (flat item frame seem to have another entity name than Vanilla ones)

Vanilla

Quark's flat ones

Technical Details (I don't know how to spoiler :D):

Tested Versions (in 1.12.2):
forge-14.23.5.2770
PassthroughSign-1.12.2-2.1.4
Quark-r1.5-130
AutoRegLib-1.3-20
(+ various mods in the first test,
second test with blank Minecraft instance, only versions above)

Thanks for the incredibly fast addition.