Thank you for your post. We already have customers using Mailchannels with Enhance for outbound filtering. You can set this as a smart host on a global or per server basis.
At the moment you need to manually ensure the customer's SPF record includes Mailchannels' SPF. We have scheduled for January the ability to set DNS templates (default DNS records for all zones) which will enable you to automate this:
Regarding the plugin, presumably the primary need is to automate the adding of customer domains to Mailchannels via their API for inbound filtering, and subsequent changing of the MX record to Mailchannels inbound servers? Would you like the ability to turn this on and off per package so you can offer packages with and without Mailchannels filtering?