I know the minimum barrel lenght has to be 16" on a rifle, either full barrel lenght or a shorter barrel with a pinned and welded compensater to make up the 16".
When the ban sunsets and we can have threaded barrels and flash supressors again will that mean I could have say an 11.5" barrel with one of the hole or slotted type flash hiders like they have on the XM77 type rifles that are 5.5" long. The lenght would then be more than the legal required 16".
In theory this sounds right but anyone know in practice would this be OK.
Was this setup OK in the past before the ban?

Even if the 1994 Assault Weapons Ban sunsets you will still have to have the flash hider pinned and/or welded onto the barrel if it is shorter than 16".  A barrel less than 16" for a rifle is a NFA item, subject to all the rules, and this is even if you have a flash suppressor screwed on the barrel.  To comply it must be welded on otherwise it will be a unregistered NFA weapon.

If you have a pre-ban AR then yes you can have a 11.5" barrel with welded on XM177 clone flash suppressor.

I'm not sure what you are asking.  Regardless of the ban, you need a barrel length needs to be a minimum of 16 inches.  So if you have an 11.5 inch barrel, you need a muzzle device that will bring the barrel out to 16 inches "permanently" attached.  If you are asking can that muzzle device be the old CAR styled flash hider, then yes as long as it's "permanently" attached.  You can have a muzzle brake/comp or fake flash hider attatched to a threaded barrel now as long as it is "permanently" attached.