The shader likely involves the stencil buffer. Which I don't think Minecraft can take advantage of.
Or at least, I have yet to see anyone use it.
If you've seen my Slidercraft video, you've seen me use it.
Though I like the "render to a screen texture" approach and overlaying that.
Either way, the trick is to render from the player's current relative perspective relative to the target location, yes. I have that working fine.
The stencil buffer solution is iffy in Minecraft, simply because it's not originally present in the rendering pipeline. Adding it can throw some interesting curve balls, especially when more than mod tries to (there are a few other mods which do).
So I'd prefer rendering the portal perspective to a texture (of about the screen resolution) and then splicing that in place. That would be a good improvement over the stencil buffer solution I have.
In general, there is only one real issue in the rest of the solution, and that's caused by the slice plane required to not render the intervening space between the player and the portal "other side." It results in blocks and entities being sliced in half. The easiest fix is probably to not render anything (dead portal) if blocks cover any of the other side. Entity slicing is less of an issue.
I think the Slidercraft mod code is up on Bitbucket under a private repo. If you have an account there, shadow, I can give you access and you can take a look.
I also recommend looking at the Slidercraft videos.