This doesn't help answer the general question of how to better promote/advertise the FOTC charters.
But the Sept. RGS Charter was intended to be tied to the National NG Convention, after all. Would the convention organizers be willing to post a more complete and descriptive ad on the Convention website? I just checked, and all that is there now is the same single paragraph blurb that was provided when that Convention site was first built, probably from January. It's somewhat down the page under "Convention" and "Railroads and Railroad Museums", so placed correctly, just not that visible. The text of the paragraph is generic and suggests it was placed before details and features of the trips were decided.
At very least, a better, updated description with costs (to show how reasonable) and a link to the nice ad posted at the top of this thread could draw more NNGC attendees to Antonito.
Bob of AZ