Your right, hype has a lot to do with it. I'm on the Scarlet Horn team and if we ever get it online I'm confident it would initially get a solid online count. Sadly, the last update to the change log was submitted by me on March 6 of last year. I'm not sure what you mean by future demand for custom OT's, but it seems to only be decreasing. I hosted a custom OT a few years ago (anyone remember Road to Zion?) and the quality wasn't even near to where the server I'm playing on now is. Somehow it still managed to average 150+ people, with a max in the four hundreds. I'm assuming its because this was back before real map servers started popping up like roaches.
While I wasn't around on my current server in the preliminary stages, I did join within a few days of it starting. The preliminary marketing was lacking in areas, but since its gone online we have created facebook pages, twitter pages, and used google AdWords, all of which were pretty ineffective. Honestly the most effective form of advertisement seemed to be posting on this forum and the OT server lists.
We have always developed our current server to be a fresh, but still familiar, take on classic tibia. It was never our goal to create a new game, there are plenty of better options for that than using otclient with custom sprites. If that's the approach you were trying to take than I highly recommend you checking out the
Unity engine if you haven't already. It makes it a breeze to create cross-platform, high quality, 3D games to be used on either mobile or desktop devices.