Yeah, aside from the month or so and probably 2 tests playing he'll get with the squad on this tour to lay a base with Schmidt's Wallabies,
the release period for the Lions tour begins on on the 1st of July, almost three weeks before the first test:
Information about the game's laws and regulations, including the World Rugby Laws app
www.world.rugby
Even if La Rochelle ends up in the top 14 final that takes place on the 28th of June, so there's no blocker for him showing up straight away (though he might be on light duties that first week). It's also a lot easier for him to keep working on getting and keeping up to speed with the team once he has spent time with them, even if he's on the other side of the world.
If we are going to look at any more euro based players I wouldn't mind Schmidt running a session or two with them in the mean time if there's any availability, but I think he'd need to pull them in on this tour for that to be worthwhile. Sio is the only one I think really should've been included, I don't really see any of the others significantly strengthening our 23.