First of all, you are correct in your assessment that there aren't really any of Pierre's elaborate constructions used directly in the scheme. He largely only lends a hand in more generic ways, e.g. by playing distraction characters or, like you said, operating the drawbridge. And I can only provide some rather speculative in-universe reasons for why this was the case, as I also think he would have made a nice contributor to the scheme if used more.
There is however one part where they use a little automated mechanism for automatically dropping down a glass of bees on the dock workers when stealing the container of granades. This might have been devised by Petit Pierre but this is left unclear and it isn't really on one complexity level with his other creations either.
But there are also other characters that have a more indirect contribution to the whole scheme, like Tambouille who's more of a mother for the group, or Placard who is more of a jack-of-all-trades anyway. It is true that Petit Pierre in contrast has a very unique and special "super power" that could have been used more in the scheme. But on the other hand his character is also the one that most lives "in his own world". He doesn't talk a single word during the whole movie and always seems a bit out of place. And this also holds for his inventions. While they are very elaborate and impressive, they are also all rather useless and more for entertainment and enjoyment, or artworks existing for the sake of themselves. So maybe it is just not in Pierre's nature to actually design something more useful that could be used more directly in the plan. But I admit that this is largely speculation (or even "retconning") on my part here.
And last but not least, the fact that Bazil's cover blew and he was kidnapped by the two arms dealers required the group to speed up their scheme and improvise a little more. So maybe there wasn't much time anymore for Petit Pierre to play out his strengths.