If you're willing to trust the variable declaration part of the PKGBUILD, then yeah it'd be easy to execute just that part. You don't even need to cut out the build() function, since executing the whole thing would only declare and not run that function. All you'd need to do is to add some "echo"s at the end of the wrapper function you've constructed, and execute the wrapper function.
That doesnt work for overridden variables in split packages because they are set inside the packaging function(s). Anything which could selectively execute blocks of code inside of functions to get the values of the variables would be far more complicated than this approach and probably far more exploitable. Even without that to consider, you cannot blindly trust the variable declaration section of PKGBUILDs uploaded to the AUR.