What jfb is pointing out is that you can do this without a whitelist regex. Create a group called something like Unrestricted. This group has no adlists, rule or clients assigned to it.
Now go to Clients and find each client that you want to control. For each one, click Add and it will appear in the section below and will show as assigned to the Default group.
To temporarily unblock a client, click its drop-down for Group assignment, click Unrestricted to add it to that group, and click Default to remove it from that group. So you're essentially swapping the ticks over. Because the Unrestricted group has no adlists or rules assigned to it any client who is only in that group has no blocks. When you want to turn blocking back on, just put them back the way they were.
Thinking about it, I guess the advantage of a regex to whitelist everything (.*) is that you can assign that rule to the Unrestricted group, and then just toggle that group on or off for chosen clients as needed, without needing to also take them in and out of Default. They can stay in Default all the time, with all its blocks, but while they are also in Unrestricted, everything for them is whitelisted.
I use the first method, but that's because I have some clients which permanently have no restrictions. If I was moving them in and out all the time I would probably use the second method and save a couple of clicks.