Named capture group is part of the pcre engine, if you can create a group (non-named) and reuse it inside the expression aka /(a)\1/ matching "aa", you can do that with named captures as well /(?<group>a)\k'group'/
This works in mIRC currently and you don't need to reference the group with \k for the group to exist, so:
If $regex* doesn't currently support named capture groups, it's pretty safe to suggest that sane people aren't using named capture groups in scripts.
in that sense, $regex *does* support named capture groups, sane people are using named capture groups in scripts. That's why your suggestion breaks compatibility.
if named capture groups would populate variables of the same names, it's very safe to say people would use this feature... enthusiastically.
The problem is not about if people would use the feature or not, the problem is that the suggestion changes how previous script will work, it's not a feature that you can use or not, you're forcing the user to get the feature. What you call safe is actually very unsafe to me since it just breaks compat.
Wanting mIRC to automatically translate named group (but even just normal group) to local variable, I can understand that people would want to do that but.. it's already possible to do that in mIRC, but you called the current way to do it "tedious and utterly line consuming", that's basically calling any script looping over a list of result like that tedious and utterly line consuming, so it should be an issue for you with mIRC as a whole and not just here.
It's extremely slow to loop over $hfind and $findfile with a while loop, that's why they have a command parameter, others identifiers returning collections like that do not have this feature, seems clear to me the way to improve this is to extend $regmlex (and $regml maybe) to support a command parameter and then do the call to /var there by yourself, which should be very close in speed to what mIRC would be doing internally and would not be breaking compat!
Of course this would be improvig $regml* themselves, you would still need support for accessing named groups with mIRC, outside of the expression.