Target specific computers with SUS?

Configuration profiles would be your best bet. Scope them to a test group or whomever first and everyone else could just get a dummy SUS URL from a 2nd configuration profile. Depending on your network, the config profiles update pretty fast so pulling the dummy URL at a later date wouldn't be that hard.

Reposado with Margarita is good for doing this. I have three separate catalogs, one for testing, one for fac/staff, and one for my labs. I use MCX/Profile to direct the clients to the appropriate catalog. Using this method, you can delay updates for your users for as long as you want.

Second looking at either Reposado or the JAMF NetBoot/SUS appliance: https://jamfnation.jamfsoftware.com/viewProduct.html?id=180&view=info (which uses Reposado) These let you create specific SUS branches and point Macs by groups in Casper to those specific branches with a policy, MCX, config profile, your choice.

@brandonpturner, point all clients to you internal SUS.

Create a policy for those that you want to test updates on, & add the following as a script before:

sudo jamf removeSWUSettings

then tick install all updates.

Enable that policy when you want to test updates on those macs.

@bentoms answer looks the cleanest. I'd like to avoid more third party apps and plugins. I'll give it a shot. Thank you all!