#nebuta

For one, it's not a regular cross but a Celtic cross. So, theming, if only for a cultural aesthetic.

Secondly, Devil Summoner demons represent modern times, and Ireland is today (read: 1997) a Christian island.

But let's see what Kaneko had to say about it, if anything:

ドルイドが人身供養をしてたんですが、そのときに10mくらい ある巨大なワラ人形みたいなのに、人を入れて焼いてたらし いんですよ。それをウイッカーマンっていうんですが、そのワ ラ人形をストレートに描くと、ワラを描くのが大変なんで、ね ぶたみたいな人形にしてみました。供養のたびに作るのは大 変でしょうね。しかも、その人形にウィッカーマンっていう名 前をつけちゃうところが凄い。その人形共々、焼かれた人が 悪霊になったっていう解釈の悪魔です。

Machine translation of the Soul Hackers no subete comment:

Druids would hold human sacrifices, and apparently they would put people inside a giant straw doll about 10 meters tall and burn it. This is called a Wicker Man, but if you were to draw this straw doll directly, it would be hard to draw the straw, so I made it into a doll that looks like a Nebuta. It must have been a lot of work to make one for every memorial service. What's more, it's amazing that they even named the doll Wicker Man. It's a demon that is interpreted as the person who was burned together with the doll becoming an evil spirit.

So no extra insight about it, but I feel fairly confident about my speculation. I also think it looking like a Nebuta is the "modern Japanese" aspect of the design made to meet the Devil Summoner worldview, and the cross is just there to signify the thing's association with Celtic regions within Great Britain that are now predominately Christian. It's basically just an accessory, so it looks more "foreign." In other words, I don't think this Wicker Man worships Jesus.

FWIW, the Celtic cross is also on the prototype design: