Just like Training, this can be a natural extension of Technical Writing.
In order to document a system (product, solution, offering etc), I need to have a very good understanding of it. So, then helping you Market it, is something that I find comes naturally to me.
In fact, I started my career in Technical Marketing and have enjoyed doing it for years.
This can be presentations and demonstrations, aimed at whatever level of audience you desire. From overviews to heavy technical. I can develop and present these materials, including associated handouts and even web content.
Of course this can be independent of any Technical Writing. I can assist just with the Technical Marketing, if you that's of value.