Does anybody know if a front BA seat belt will fit into a AU sedan.

More than likely but I don't think it would match the Buckle part. They mount similar but I think that is where it ends. I would stick to your AU one, BA's have a tendency to get a little sloppy over time.
We have 8 utes at work that have had the Belts replace once and 2 of them had to be done a second time, got too sloppy and wouldn't retract properly.

Remember, these are safety items and could be the difference between you going home in a car or a a***?

BTW, I meant stick to using a AU one, new of course

If you would like I can get a price for you on a NEW AU one? Just leave me your VIN and Iwill do the rest.

Check your PM mate