I think the biggest thing is what would constitute 'national' service? I don't think it necessarily has to be military occupations only, it could encompass many areas that are outside the traditional grounds of the military. Some military roles can be options for sure, but each role has to be considered separately, because a year or less in many trades is pretty much only a drain on resources.
A national forest fighting service is one thought, people could be trained up fairly quick to be valuble in that. Could also be used on the flip side to help replant forests cut down, or a quick response team for disasters during the winter, like when Toronto asked for help with their bad storms, Quebec Ice Storms, Snowmagedon in St. John's. It could help take the burden off the regular force and let them focus on the more traditional military roles.