by Ken Task.
Yep ... 2.7 was when the cli backup.php script first appeared - none for 2.6.x. Don't have a functioning 2.6 up anywhere to do a quick test soooooooo ....
And, yes, you could try that ... copy a 2.7.x backup.php script into admin/cli/ of the 2.6.
And try it ... think I'd try course id 1 (that's the front page - which you can't restore anyway) cause it's generally small as far as a backup is concerned *and* use the option to save into destination directory .. *NOT* filedir. Make sure your 2.6 is set to use what it called 'experimental compression' - gzip instead of zip. That will make your manually created backups forward compatible - or less prone to issues ... me thinks.
If that doesn't work, get moosh ... but here again, one might need to install a version of moosh that isn't too high ... can't tell you which one, but here's the link:
https://moodle.org/plugins/pluginversions.php?id=522
I can re-call tinkering with that ... I did have multiple versions of moosh on same server ... fairly easy to do because the command you create to use is an alias ... had a 'moosh' which pointed to the highest version ... and a 'moosh29' which was aliased to the 29 version of moosh.
The actual moosh directories changed slightly ... moosh and moosh29.
'spirit of sharing', Ken