90% of times when you need to EQ it's because the instrument's body, amp, cab or microphone did picked up certain frequency range too harsh or too low. In such situations the problematic frequecies are not really pitch dependent = they do not change with the note the instrument is playing. Normal static EQ is a way to go in such situations.
Then there's that 10% times, when certain harmonic(s) of the instrument cause problems and they naturally change with the note, making them really hard to pinpoint using static EQ (basically you have to compromise between fixing the problem on some notes and needlessly damaging audio on others). In that case pitch-tracking EQ is the tool to go.
Some time ago (like 2 years back) I've made similar plugin in synthmaker, but never really released it, nor seriously tested it. It is definitely not "must-have" type of plugin, but it definitely has its uses. I may send it to you to try it out if you want...
