Don't know if it would be acceptable to everyone or not, but I'd run it like that. Never really read a spec on it as far as minimum engagement.
Edit: Being curious by nature, I googled and found this post on another site. It was posted by "KeislerJeff" and I have to assume that he has some connection with Keisler Engineering, the transmission guys.
"As a rule of thumb, you need a minimum of 3/8" engagement of the input into the pilot bearing in order to properly support the input shaft. Anything less, and you run the risk of damaging the input, anything more, is just icing on the cake.
He went on to recommend a roller bearing instead of a bronze bushing, stating that they want to retain the centerline of the input shaft without the "wallowing out" of a bronze bushing, which allows the shaft centerline to wander.
There are two sides to every story though and I would prefer a bronze bushing due to the fact that even when it's worn out, it will still support the shaft to some extent. When a roller bearing goes out, it's "get your tools out" time.