OPI or Illegal Participation?

[mikesears]

Originally posted by Zeke5
Play: (Federation)
3rd and 7 from A's 40. A86 is running a deep sideline pass
route where he, under his own power, steps on the sideline
at A's 43 and continues downfield where he catches a pass
inbounds and is tackled at B's 40.

What penalty do you assess A with? Illegal participation
or offensive pass interference?

I've heard it both ways from the guys in my chapter. I think
they are both too drastic (15 yds), but at least the
participation foul doesn't carry the loss of down.

I don't think this is OPI because a player who starts a down as an elible remains eligible during the entire down (even if he goes OOB.) This should be an illegal participation foul from my understanding of the eligability rules and from the basic principles.