It sounds like a feasible option and the concerns raised with the metro CFS crew training would be no different from training a new member. The experience level that they would miss out on like a normal brigade member going to jobs over time would be a concern, which could be easily covered by splitting these members up through strike teams. Similar to what some groups are doing now with composite strike teams with an appliance covered with fire fighters from that group’s brigades.
Also they could help brigades/ groups that are struggling with numbers on active stand-by days.
The suggestion of having them man busy urban fringe brigades during the day I tend to disagree with due to the fact that it would take some time to get them to a decent level of training to cover structure fires and road crash rescue which is the highest amount of jobs those brigades respond to. Also I think if an urban brigade is so busy that it requires full time crew there I say make it paid, I understand we are volunteers but we are not free labour to be exploited. Plus I think they would quickly loose interest sitting around a station all day maybe going to one or two stop calls or nothing as our urban fringe brigades are not super busy.
In closing I think it would be a great idea for some really keen people. As I know my station turns some people away as they live to far away for a decent response time. A good idea!