Well, lots of con people clip them to a belt so they hang at the right place (except for "those" types of cons where it would be acceptable to walk around sporting an obvious butt plug)... They make that kind too, it's just not as fun to link onto random subreddits.
But you may be on to something... Attach it to the loop on one of these, and it would look like it was sprouting out of your tailbone rather than your taint.
also to answer /u/FHBruce 's question, there is actually a common sex toy that fits his description.
It's called an Anal Hook It's an amazon link but NSFW just in case.
Its just a standard butt hook.
Does your grappling hook look like this?
Oh, my sweet summer child. We have such sights to show you!