Nope, you can't do hashtags in CWs, and it's been closed as an issue, presumably as a wontfix:
@wohali Nope, you can search for hashtags hidden behind CWs. If you search for #nangartag, you'll get the test toot I made a few minutes ago to verify this.
@nangar I meant in the CW itself. I know you can search for hashtags within the content of the CW-hidden text.