Thanks for that. I had no idea what Hydrogen Audio was, and assumed it was a software or manufacturing company. I'm impressed by the caliper of participants, and now have them bookmarked.
I just now finished reading your thread, and have a few comments:
First, although your question was quite general, the responses were more narrowly focused on IIR filter-based equalizers. IIR filters make for a good example, based on their non-linear properties, but it does tend to slant the responses. The equalizer software mentioned, particularly the Sonoris Equalizer software, are more "applications in a plug-in" rather than a single "effect" in a plug-in. In that type of situation, it is indeed desirable to have the sample-rates controlled by the plug-in, as the final result is very dependent on the sample-rate. So, in that context, I would agree that up-sampling within the application would be preferred.
There was only one response that referred to non-linear effects unrelated to EQ (post 4: compressor/expander and wah-pedal), and that was more the application I was considering when I mentioned that sample-rate conversion should be done outside
the plug-in: a series of effects in a chain, typically used as inserts.
I also noticed that no responses addressed FIR filters, where benefits can be derived from down
-sampling as well as up-sampling. Bilinear transforms are unnecessary with FIR filters, because they have no frequency-warping, so that benefit of up-sampling doesn't exist for FIR filters.
One thing that the thread pointed out, but could have stressed more, was that resampling
would not be inferior to having the original recorded
at the higher sample-rate. Although resampling can add a insignificant amount of noise, so can recording at a higher sample-rate. This is a fact that I often forget.
All around, it was a very informative thread.