tl;dr Thanks for coming to meta to ask before posting. It doesn't seem likely this will work on our site, but since you asked, here are my thoughts below, which include some possible, but not guaranteed, ways this might work.
Reviewing is off-topic
Reviewing the cover letter would be off-topic for this site, simply because it wouldn't fit the Q&A format that is part of all Stack Exchange sites. It's also difficult to formulate specific questions about your cover letter that don't end up with the community reviewing the whole thing or taking something out of context.
However, there is a way in which you can use the platform to seek assistance. As someone with freelancing experience, you may very well have suspicions as to what might or might not be wrong with your cover letter.
Specific questions about cover letters could be on-topic
Try to see if you can think of a question about cover letters in general. Mention your target market, a brief paragraph about the type of work you do, and the demographic of the client. For instance, "What should I include in a cover letter when targeting a client with 20 employees, who is in manufacturing, and who runs a flat, empowerment based organization with little hierarchy?" would be something specific.
If you have a specific question you could ask that could lead to answers which explain why and how, then this could work.
I can't promise it will work though. It's possible the post may be put on hold, either by the community or a moderator, until the post is edited to be on-topic. If that happens, it's not a reflection on you, and try to be open to feedback from the community on how you might edit the post to be on-topic. Look at this as an experiment to see what's possible and not possible on our site. Hope this helps.