Is there any reason you're not using filtercoeff~?

Only that I wouldn't know how to get it to behave the same way as the peq2~ patch.

Perhaps you should look at gen~ for your implementation. The syntax will vary slightly from the expr syntax, and it's all single sample (so there are no trigger objects), so it'd require a little jiggering.

I've tried building it in gen~ a few times but can't seem to get it working properly. I will try again.

You might have greater success by sharing the source you're working from and having another set of eyes on that.

Yes, you're right, I should have done that from the start. Here's my latest attempt with still no success.

Got it working!