No, the typical manual gas valve can't be converted to operation by a switch.

And frankly, unvented equipment is very likely the wrong choice for your cabins. Your guests aren't going to follow the lengthy list of warnings that come with all unvented equipment, and especially if they use them for heating they can be killed or injured by such equipment.

And please don't tell me you have CO detectors, which aren't adequate protection against the hazards of unvented fireplaces.

You should consider the very very expensive lawsuits that you invite by supplying wholly unsuitable and risky equipment for your visitors to use.

As a repairman for a natural gas utility, I packed quite a few people off to the hospital for carbon monoxide poisoning, and shut off a good many hundred of gas appliances that were hazardous to operate. Unvented equipment is a foolish risk to take unless you can be SURE that ALL the warnings included with such equipment are understood and followed by all the people using it.