Domain Mapping is a great plugin.
You have already integrated eNom for domain purchasing and mapping.
However, why not integrate their SSL reseller ability too.
People can 'upgrade' their site to be SSL, the domain mapping plugin goes off to enom and gets the instant SSL certificate.
Meanwhile, it would be the domain mapping plugin that handles all the http to https redirect stuff.
It is also not part of the usage instructions for the domain mapping plugin page. I have tried to find instructions to implement multisite with domain mapping and found these convoluted topics:
Any +1's ?