Turn on Back To My Mac via a Script or Command Line

There is no supported way to do this. Having said that, you can do it using scutil:

louis@Arios:~$ sudo scutil 
> set Setup:/Network/BackToMyMac
> d.add <YYY>  <XXX>.members.btmm.icloud.com
> set Setup:/Network/BackToMyMac
> quit
louis@Arios:~$ 

Replacing <YYY> with the UID of the user account on the system (for most people with single account that is 501), and <XXX> with your iCloud account number. If you don't know that you can check your dns-sd registrations:

louis@Arios:~$  dns-sd -E
Looking for recommended registration domains:
 DATE: ---Wed 22 Aug 2012---
 9:11:04.789  ...STARTING...
Timestamp     Recommended Registration domain
 9:11:04.789  Added     (More)               local
 9:11:04.789  Added                          icloud.com
                                             - > btmm
                                             - - > members
                                             - - - > <XXX>

The last line will list your iCloud user number. I think they are all 8 digit numbers, at least mine is.