This was something I had on my mind since some time ago and only bothered to try it out today. As a lot of you probobly know, I was one of, if not the first user on this site to include unportable Brick blocks in a custom tile set (and with SE I'm to do it again with mirror blocks). And since any block that has an entity placed inside automatically becomes one of the Used Blocks in the default SMB tile set, which are portable, we have this scenario:
Here we have three rows of unportable brick blocks with the "Bill End" entity placed inside every one.
So we whack four of them on the bottom row, turning them into Used Blocks...
...And thus, enabling them to accept portals.
If you want to see/try them for yourself, you may refer to this small tile set I just made:
