I am making this feature request here on the forums, which I don't usually do, because I have only ever seen one gig file that needs me to do this.
The Tajiguy Gigatron is a GIG/GIGA file that allows you to, in some players, change instruments when clicking on the first 12 keys on the keyboard. However, LMMS does not have this functionality, and while I would like it, it seems an absurdly disproportionate amount of code (no matter how little) to add, just for the few files that have this ability. Ideally I'd put it on the github, but I'm not sure if it even makes sense to ask.
I would like the ability to perform keyswitching in gig files that support it. They aren't multiple patches of the same instrument, they're all WITHIN the single patch.