I'm not sure if this is something for WE as well as WECUI, but //sel convex is partially unsupported. The view given by CUI looks like a regular cuboid selection.

I'm attempting to work out the Forge problem, and I can confirm it's on the Forge side. However, I can't get the custom S3F packet to pass data from Forge to LiteLoader, do you know if there's a specific way I need to send it?

I have no idea how plugin channel messages are handled in Forge to be honest. The protocol is quite straightforward, you simply advertise that you want to send messages on a particular channel which will cause the platform to send a REGISTER packet to the client. Then you are free to send messages on that channel.

Huh, I think it should be working but something isn't getting through. Is there an easy way to dump the incoming packets on LiteLoader?

Just subscribe to the channel, any mod can subscribe to any channel (except for REGISTER, which is reserved) so writing a mod to dump the incoming packets is trivial.

I'm able to use Forge to see the packets, but LiteLoader just doesn't even call the onCustomPacket.

And this is on WorldEdit for Forge, MC 1.8, with WECUI.

Since this works fine on Spigot as I already mentioned I'm going to assume this is an issue with the forge implementation and isn't a WECUI problem.