Something like this would be great, "BUT" I just want to point out that allowing an IP through the firewall is incredibly dangerous (Even for standard customers).
Allowing an IP through the firewall would permit unrestricted access to the added IP, which means they would not be blocked if they decided to dictionary attack your server, or try to brute force your MySQL / Emails / SSH, etc,.
If someone takes this on, consider adding options to only allow unrestricted "Specific" areas / ports. You can customize what ports / areas the customer can access via the CSF Allow list.
Just my two cents. 
This will only allow for logged in users to remove a temporary or permanent block they have received. They will still be blocked again if repeating the offense and it will not open any additional ports etc. Just removing a block. =)
Typically customers get blocked for entering the wrong password when logging in to cpanel/webmail, changing the password for their email account and forget to change a client, suspended customers who have their email client set to receive mail, trying to log in to wordpress too many times, modsec false positives, etc.
This accounts for a huge portion of our tickets and clients get anxious if we do not resolve immediately.
I have seen a few whmcs csf unblock plugins, where some of them have added security as of how many times you are able to unblock yourself etc. Might be a good addition to the linked script, but I would be happy with the functionality as is or with the added function to send a mail with information every time someone is unblocking themselves.