Thanks for the feedback!

It would good if we can get /promoted displayed somewhere on steemit.com.

I agree. I don't know why it's hidden, but hopefully that changes sometime. Personally, I adjusted my "New Tab" page years ago so that /promoted is my landing page now, but that doesn't help for other curators. ;-(

It might also be useful to remind people how to promote their posts - I keep forgetting!

Yeah, I wish this were documented in some permanent location, FAQs or something (or maybe it is, and I just don't know where). It disappears into the ether if we just keep publishing it in posts & replies. As you rediscovered, it only works with SBD, not STEEM. And the memo field has to be in a very specific format:

@user/permlink

Nothing else works for whatever reason.