It sounds cool to add that large arachnid, but I'm curious where?

Without any new scenes being added to ESB:R (so we've been told) and without it interfering with what's already happening on-screen, the only place that comes to mind where it could work seamlessly is in that wide shot of Luke's X-Wing after he crashes it - you might catch a glimpse of one scampering off-screen to the right. Otherwise, there's honestly no reason to see them.

I can definitely see some other little alien creepy-crawlies/fliers (besides earthly snakes) moving around in a few shots though, mainly in the cave of evil or any other shadowy/dark shots.