Montoya can take some Ligier info back to Penske, so it was probably not hard to get Roger's blessing. In fact, I would not be surprised if this ends up being a reconnaissance mission for all-out Acura assault on Lemans. It's a good time to be a sportscar fan.
Whether or not Montoya can actually be in the fight remains to be seen. The Ligers were miserably slow at Lemans last year, mostly because they had so much down force that they were getting killed on the straights. The FIA has given Ligier a "joker" to revise their car, and we won't have much of an idea how that does or doesn't help until they run at Spa on May 5th.
Worth noting is that Spa is the same weekend as IMSA Mid-O, so Montoya probably won't get much set time in the Ligier until they actually get to Lemans. I'm pretty sure he will figure it out quickly. If the car is good, they will be in the fight.