Most of the places that have such "engineer for an hour" programs do it at the end of the day, after the excursion operations are concluded. This precludes the expense of firing up an engine and associated costs, but generates some revenue from a locomotive that would otherwise just be sitting there.
But say the C&TS eventually gets enough locomotives going to have a "protection engine" fired up (like the Strasburg does), perhaps at each end of the line. Why not put them to work earning some revenue?
If some track were laid at Como and an engine brought in, the idea could work there also, bringing attention to the roundhouse restoration project and raising funds in the meantime. Wouldn't you rather pull the throttle on old No. 71 than see it rusting by a casino?
The idea was proposed and could have worked at Hugo also, if the feuding locals hadn't killed the restoration effort. Lay some track, get UP to donate a GP9, and get on with it. Diesels cost very little to operate and don't need much maintenance.
Thus far, those groups that have begun engineer experience programs have not reported problems with insurance, nor anything else of a critical nature.