Here's a hook to record WHMCS client area logins, along with IP address and RDNS host:
https://github.com/damianharouff/whmcshook-recordlogin
Put it into yourWHMCSdir/includes/hooks/ and it should begin showing logins under the Log tab on the client's profile. Example:
If you don't like the name of the file or function having "ipxcore" in it, feel free to change to whatever you'd like.
Also, thanks to @serverian for helping me with a hurr-durr problem with it before release.