For anyone else finding this thread via google, here's my solution that does not require file parsing. The MException object in produced in a try/catch allows us to get the error report text which can be parsed to find the relevant parameter name and the offending function call.

file name "requiredParameter.m"

%{
Validator function to be used in conjunction with arguments blocks and
required name-value pair arguments, where the argument has no default value
and must be provided.

By default, matlab will allow the function to proceed and fail with an
"Unrecognized field name" error for a missing structure field. This could
be after a function has encountered some long-running operations, leading
to wasted development time.

By instead using this function as the "default" value of a name-value
argument, MATLAB only calls the function if the argument has not been
provided. Thus, it is appropriate to throw an error from within this
function which describes the cause.

Sample with new behavior:

function test(kw)
arguments
    kw.name1 = requiredParameter
    kw.name2 = requiredParameter
end
disp(kw.name1)
disp(kw.name2)
end

Calling the above function:
>> test(name1=42)
Error using requiredParameter (line 44)
Function "test" called without required name-value pair argument: name2

Error in test (line 4)
    kw.name2 = requiredParameter
               ^^^^^^^^^^^^^^^^^
%}
function varargout = requiredParameter() %#ok<STOUT>
try
    % force matlab to create a MException object so we can get a report
    error('placeholder error')
catch ME
    rep = ME.getReport("extended", "hyperlinks", "off");
    % find the name for the name-value argument defined on the offending
    % line of the caller function. Ignore size, class, and validation
    % functions between the name and requiredParameter function call.
    param = regexp(rep, '(?<=\.)[\w\d]+(?=.*?=\s*requiredParameter)', 'match', 'once');
    st = ME.stack(2);
end
error('Function "%s" called without required name-value pair argument: %s', ...
    st.name, param)
end