No what I'm talking about are complaints about (and this was from G-sync users) that they couldn't set a custom refresh rate to something like 100Hz or 110Hz in the Nvidia control panel on a G-sync monitor to better match their GPU power FPS and cap it. Maybe something's changed or they didn't know what they were talking about (or doing).
I don't have one so I can't comment. I overclock my 1440p monitors to 75Hz (Dell) and 90Hz (Crossover) and cap frames accordingly, but just have never been clear on what that meant to a G-sync monitor that advertises 120Hz/144Hz capability.
I'm not sure I fully understand your concern but, if I may, I'll give it a try.
As a user of Asus PG278Q (with G-sync) for a year now, I can tell you this much:
G-sync, much like FreeSync, works within a frame rate range, depending on the monitor and not the adaptive sync technology behind it, in my case within 30-144Hz. Between that frame rate range, the refresh rate is variable and depends on how many FPS your GPU can push.
This is where the similarities between the two stop because outside of that range the two technologies behave differently. Below the minimum range, 30 FPS in my case, the G-sync module automatically displays the same frame twice, making the frame rate appear double than what it is and the gameplay feel smoother. At the other end, G-sync module automatically caps your frame rate to the maximum refresh rate of your monitor (144 in my case).
That being said, having nothing to do with these adaptive sync technologies, Radeons do have a frame rate target control feature in the Catalyst control center (or whatever it's called nowdays) for power savings reasons, feature that you don't have as a Nvidia user.
Now, regarding your concern, a custom refresh rate simply defeats the purpose of having an adaptive sync technology and, outside of power savings reasons, I fail to see how a custom refresh rate target would help since G-sync (and FreeSync for that matter) already cap the refresh rate of your monitor "to better match their GPU power FPS".
If you prefer a custom refresh rate, you can chose do simply disable G-sync and set your (G-sync enabled) monitor to a fixed refresh rate (in my case I have the following options: 24, 60, 85, 100, 120, 144 Hz).
I hope that was helpful.