Yep, a guitarist as a hobby - I don't play on a scale that would warrant a large format digital console capable of these processes, unless I steal one from work and hire a van to shift it (it has happened). For a day job I'm a Systems & Support Engineer for a console manufacturer. I count myself fortunate to work with a lot of live, broadcast and studio engineers internationally on tours, festivals, concert series or other live-to-air events. Primarily I work through the preparatory stages of facility commissioning, tour rehearsals/first gigs, provide operator training, often through first shows or station broadcasts as well as offline events. It's the broadcast work that has drilled mono-compatibility into me, but live pop/rock/world music tours where stereo widening techniques seem to be deployed readily to, at least in part, clear up busy centre-focused mixes and make a narrow stereo mix seem wider than it is without hard panning.
I don't see the workflow as just for guitar; it's usable for any single point source sound, and yes the context of an overall mix would be considered. It's also born out of practical use and experimentation rather than just theoretical; I've either tinkered with it myself when mixing or worked through it with others who are far more talented than I, putting it to use it on backing vocals, small brass sections, widening mono effects returns, a host of world music instruments or in some cases anything in a busy centre-focused mix that can afford to be lost in the mono down mix due to the cancellation. This is where the centre channel can help - there aren't many guitarists that would agree they are sonically dispensable. Typically, it's the sort of workflow question that emerges when a tour has gotten underway and engineers look for subtle refinements they can add in now that they are recalling the mix settings from previous nights and not having to start from scratch each time.
Having settled on this mono+sides workflow when required, I'm still trying to work out the best way to combat the more obvious remnants of the Haas effect, and have been trying out all-pass filter effects to rotate phase of particular frequency bands on just the early channel. I got poor results when trialled at a tour rehearsal though as it seemed to exacerbate comb-filtering effects. An engineer at a theatre I regularly visit mentioned he used the HPF trick on the early signal to try and improve things. This is different to 'usual' widening workflows I've seen, where a low pass filter is used on the delayed signal to keep the effect without adding to the 3k and above busy part of the mix. I'm not convinced of that method, but it seems reasonable though that if you high pass the earlier of the two delayed components (left through this example) you can reduce the chance of comb filtering with the centre component or the right channel, and the inter-aural amplitude difference favouring the R component will slightly offset the effects of inter-aural time difference favouring the left. To my ears, this is what seems to happen. These later conclusions are indeed more 'theoretical' explanations to what I perceive, so it would be good to get someone else's opinion. Worth experimenting with though. I'm by no means an authority on the subject of widening though, having 'found my way in the dark' to get to this point, with some experiments and collaboration. This is why I mentioned a PA systems tour tech or a seasoned FoH engineer would be a good person to comment, given that they are either having to solve phase incoherency or deliberately introduce it for creative effect on a more regular basis than myself.
It's quite amazing to me what the Kemper can already do. It's this ability to break new ground that makes me keen to throw further ideas into the ring - I shan't be offended if they are deemed as not worth pursuing, just trying to offer something that may be of some use.