... The MQA targeted version of test tracks should be mastered to optimize the signal time-domain, where as the RBCD and High-Rez targeted version of those same test tracks should be mastered to optimize the signal frequency-domain, which is how they are typically mastered for commercial release. ...
Tracks are never "mastered to optimise the signal frequency-domain". They are mastered to sound how the mastering engineer and producer, at a minimum, want them to sound. For previously mastered releases, this means the way they sound through a traditional "frequency domain optimised" chain. Processing them with MQA results in a variation from the released version. The record label doesn't go back for approval to the people who signed off the original though, they just release the MQA version. This won't matter to the vast majority, but a significant proportion of audiophiles won't be happy.
As you point out, and I have also pointed out elsewhere, mastering for MQA requires an MQA chain in the mastering studio. Having to send a mix away for MQA to add the secret sauce, receive and listen to the result, tweak and repeat is too cumbersome a workflow. This also implies that the secret sauce flavour, namely the choice of leaky filters that results in the maximum aliasing and imaging with the minimum of audible damage, should be under mastering engineer control. Of course, it also leads to the likelihood that a mastering engineer will prefer the sound of one of the less than optimum filter sets...