Comments are for discussing the post that you are commenting on. Commenting on an unrelated post in order to gain attention for your question is an abuse of the commenting system. The help center's explanation for the "comment everywhere" privilege says
When shouldn't I comment?
- Secondary discussion or debating a controversial point; please use chat instead;
Additionally, there are some relevant posts on Meta.SE about contacting users:
How do I contact other users?
Any way to send a personal message to another user? (answer: no, and that's by design)
The first of those two posts recommends using the chat system and notes that
One might use comment replies to notify a user of a comment. But remember: even comments should be relevant to the post, and can be flagged as noise if they are not, so this is not recommended.
In my experience TeX.SE is one of the most friendly and welcoming sites on the SE network, so you probably won't get into too much trouble using comments for this purpose here. Nonetheless, you should be advised that
- users are well within their rights to flag such comments for deletion (e.g. as "not constructive")
- moderators might decide to take action if you do it too much (e.g. the users you contact start to complain)
- you might be more likely to get into trouble on other sites in the SE network that are less friendly
If you decide to use chat, your best bet is probably the TeX.SE main chat room. From there you can link to your question and possibly ping a specific user (post a chat message with
@username, though that only works if the user has been in chat recently). You can also invite a specific user to a chat room from his/her chat profile.
If you're looking to get attention for your question, the system you're really looking for is the bounty system. From the answer to the Meta.SE FAQ on the bounty system (emphasis added):
A bounty is a special reputation award given to answers. This feature was designed to motivate answerers, and help questions get the answers they deserve.
You have to spend your own reputation (at least 50) to post a bounty, but in exchange your question is put on the highly visible feature tab for the duration of the bounty (up to 1 week). Furthermore, as @Christian Hupfer mentioned, you might be able to convince a kind high-rep TeX.SE user to offer the bounty for you.
setspace.sty- what you are asking in that question is far from trivial and the comments there explain this.