So I've found a group of extremely niche tags that seem to be related, but I'm not sure which should be aliased to what.

snake_impalement seems to be a popular one, but its wording makes it less-suited to covering something that isn't a snake, like it claims in the wiki.
Related tags I've found on similar posts are double_threading, tentacle_link, throat_transfer.

I'd do it myself, but I'll probably forget about it because I'm currently conducting a dragnet for old tags that aren't indexed.