Modpack version
1.72b

Issue
The OpenBlocks Tank is not showing in JEI when searching for it with either "tank" or "@openblocks", but it's still possible to find it by looking at the usage of obsidian... Is it intended to hide it?

Yep it is intended, showing it tends to cause quite a bit of lag.

Hmmm ok I see, but this is a cheap tank and it's really useful imo, could it be possible to replace its 3D model in JEI by a 2D image within the modpack? Or is that something that only OpenBlocks developers can do?

No, that would require you to turn it into an item, and that kinda destroys it's purpose. It's craftable even though it's not in JEI - glass panes in a plus sign with obsidian around it.