Yes, you can do it in frontmatter, or by having a paragraph which has tags only (see Objects#Tags).
It definitely would be possible, but I don’t think it’s worth it now, since we don’t have autocomplete in command prompts, I just described it briefly towards the end of that post: Folder auto complete - #5 by Maarrk. Since the completion in frontmatter could be improved (#736), I think that just writing #trash in a paragraph is the most convenient solution for now.