Games : Creating a minecart spawner that had blocks riding them

I know how in 1.7 you can summon a minecart that has a block "riding" it, as in you can change the block's height above it, what block it is, etc.

I also know how to make a minecart spawner.

How can i combine the two in order to create a minecart spawner with a block riding on the spawned carts?

I saw this on a server, it's a teleportation room and when you hit the button it puts a spawner below you, and you see this block whirl around you. Any ideas? This may also be part of a plugin, idk