This is a late answer but I hope it helps someone. You could easily use the same technique in item #1, except you need to add another validation on the number of items in your vector:
from rcollyer’s example:
namespace po = boost::program_options;
po::option_descriptions desc("");
desc.add_options()
("opt", po::value<std::vector<int> >()->multitoken(), "description");
po::variables_map vm;
po::store(po::parse_command_line(argc, argv, desc), vm);
po::notify(vm);
vector<int> opts;
if (!vm["opt"].empty() && (opts = vm["opt"].as<vector<int> >()).size() == 2) {
// good to go
}