Bukkit

It is possible to create diagonal portcullises. They are essentially a staggered series of two block wide portcullises next to each other. They each have to receive a redstone signal separately, and the challenge is to do so without accidentally creating unwanted redstone connections that will confuse the plugin and cause the portcullises to work incorrectly.

There are two working examples of a diagonal portcullis on the mc.pepsoft.org demo Minecraft server.

In practice, the only way to do it is to have the power come from alternating sides of the portcullis. This is the correct way to do it; each individual portcullis, together with its redstone signal source and the frame block through which it receives the redstone signal, has been marked with a coloured rectangle:

You would have to hide this mechanism beneath the floor or above the ceiling (probably best) of the portcullis, otherwise the redstone and frame blocks would be in plain sight and in the way.

It's easiest to choose the height and length of travel of the portcullis such that it is in contact with the frame blocks through which it is receiving the redstone signal both when it is fully lowered and when it is fully raised. Otherwise you would have to build two copies of this structure, one to raise the portcullis and another to lower it.

Here is an incorrect way to do it. Note that the redstone accidentally creates a portcullis which crosses and overlaps the others, almost certainly causing the end result to work incorrectly. If you made the diagonal portcullis larger, you can see you would create even more of these accidental, incorrect and superfluous portcullises:

Note that both these examples are incomplete. They have only a one high layer of fences, and they are missing cap stones to block the portcullis from rising all the way to the top of the level.