Outboard Modification of Instruments

I agree, the focus really should be on getting the sounds you want in the first place rather than relying on MuseScore to alter them. After all, the guitar effects in question are rarely just set and left alone - you hit pedals to turn them on or off over the course of a performance. So you'd also have to extend MuseScore to have a way of adjusting the controls while the score plays back. I would imagine there are MIDI sequencers / audio editors that might provide a feature like that, so exporting to MIDI then using one of those to apply your effects seems like one possible way to go. But in MuseScore, the only way to do that would be to switch between different existing patches in your soundfont, as is currently done to switch between arco and pizzicato for strings, muted versus open for brass, etc. I'm not at all convinced that guitar effects should be treated any differently. If you have a soundfont that includes the sounds you want, then the existing mechanism allows you to switch between them via staff text just as is done for strings & brass.

Not that there isn't room to extend MuseScore in this way, but it seems an awful lot of work for something that can rather easily be achieved already using either of the methods I mentioned.