Are you using CatTools version 3.6.0?
Yes I am just upgraded today because I heard I was able to do this in this version.
So I just finished creating my own Custom template for NXOS and got it to work.
rgIgnoreText(3) = "^" & "!Time:"
was added. Also changed a few things to get to work.
Did not do anything to fix Brocade timestamp and is not showing as Changed in the log now.
Was that do to the upgrade to 3.6.0?
I believe that v3.6.0 should already (out the box) handle the "!Time" config line for the Cisco Nexus (Cisco.NXOS)as ignore text in the Device.Backup.Running Config activity, so should be no need to create a custom template.
You may want to give this a try so you can then use the Cisco.NXOS device type as normal.