I was not able to find some official documentation explicitly stating that @pings from posts do not work - the closest post I found was this: Can I ping any user I want to take a look at my question?
However, I believe that several users can confirm from their experience that notification from questions/answers do not work. (Moreover, in the case described by the OP where the username to be pinged does not even occur on the page this could lead to pinging several users having same or similar username - which is probably undesirable.)
If you want to test this, feel free to "ping" me in your question and if you also want to test answer, perhaps you can use Formatting Sandbox.
EDIT: I have now added @მამუკაჯიბლაძე to my answer. You can check for yourself that while you were notified about posting answer, you did not get an additional notification after my edit which included @username.
However, in the situation you described, I think a comment like this would be ok: "@username I have asked a question related to this one (with adding link and perhaps some more details on how the questions are related). I would be grateful if you could have a look at it."
This should definitely not be overused - comments are primarily intended for things related to post where the comment is posted. And it would probably be considered impolite to ping somebody simply because they are expert but had no previous interaction with related questions of yours. In short, you should use your own judgement to decide where is the boundary.
And of course, if you stumble upon the user in chat, you can ask there. If this particular user frequents chat, you can even invite them to a chat room. It will create notification and the user can decide for themselves whether or not to accept it. (Again the caveat that this should not be overused.)
I will add - perhaps as a piece of MO history trivia - that there used to be tags ask-johnson and ask-noam. They are mentioned in this answer and in the comments you can find links to some related discussions. The tag have since been removed.
Of course, adding the tag did not create notification, but it increased the likelihood that a particular user looked at the specific question. And this maybe shows at least a bit that using correct tags can improve the probability that expert in a particular area will look at your question. It seems quite reasonable to assume that many users look mostly at their favorite tags and at the tags from areas they are familiar with.
EDIT 2: Since I see that you have tagged your post as feature-request, let me comment also a bit on this. The question whether or not this type of ping works is in my opinion closer to support than feature request. If you really want to make this into a feature request, you should definitely spell out more detail. (For example, how would you deal with several users pingable by the string used. For example, writing @Martin would match many users on the site. Even if this feature would be restricted to exact username, duplicate usernames are not disallowed, so there is a possibility that this could ping more than one user.) Another question is whether such a feature question should be posted here or on the main meta, since it would influence the whole SE network. But I guess from your comment that you are probably no longer in favor of of implementing this feature. ("Thanks, this is what I needed - also in the sense that now I understand why there is no such feature: to avoid too much vanity pinging.")