In the example I posted I never directly set a string value for the "filenameOscA" channel. Something odd is going on with
chnset but I can't seem to make a minimal example. Time for more digging..
 After some extensive tests it seems that string channels cannot be updated at perf-time using the host API. I'm pretty sure we should be able to do this. I've posted an issue on the Csound github repo. Let's see what they can do.