Hmm, haven’t really seen the need yet.
Usually, for a pre-meditated solo boost, I insert an EQ that both raises the volume and at the same time sharpens the sound to cut through better. On solo, I simply remove the bypass from this plug.
For more ad-hoc solo-boosts, I use my “solo volume” slider on my keyboard. All the solo sounds in my songs get routed to a “solo volume” rack, which is controlled by a fader via a binding. This solo volume rack is initialized to 0 dB for every song. The fader normally sits in the middle at a CC value of 64; when I see the need to push my solo a bit, I simply push this fader up a bit (to taste) and back down to the middle after the solo. Easy enough. Same if I need to do an ad-hoc piano solo: I just push my “main keys volume” up a notch - and need to be sure to pull it back down after the solo, which I frequently forget; I SOOO like the sound of my piano
Since everything arrives at these summary faders pre-mixed (rack gain), this is effectively a relative volume increase/decrease.
So for my requirements, I’ve got all I need; not sure if I would actually use a solution like you describe. But of course having it in the ever-increasing arsenal of Cantabile’s little helpers would never hurt… And who knows what creative uses someone like @dave_dore or @terrybritton will come up with