[LAU] Filter envelope

Hello I'm looking for a resonant filter that's switchable between lowpass, bandpass and highpass and which has a traditional ADSR envelope controlling it. I'm wanting something that can be used in JACK Rack or something like that, or which could be used as a plugin in Ardour, in order to process incoming audio in real time. There are plenty of LADSPA filter plugins, but they all seem to be either static or swept by an LFO or similar. I thought someone, at some point, might have made a filter and envelope triggered by an incoming signal, in the way that a synth's filter does. It looks like all the building blocks are probably there in various LADSPA plugins, but I can't figure out how you'd connect them together. I'd also consider a hardware solution (but only a cheap one, as finances are very tight at the moment) and in some ways it would have some advantages. I thought there was bound to be a guitar stompbox that would do that (and no doubt a cheap Behringer knock-off version as well!), but so far I've not turned up anything suitable. Does anyone have any suggestions -- either for software or cheap hardware -- that would help me achieve what I want to do? Thanks in advance Q