Yes, I'm well aware of restrictions and inherent contradictions of the project; like mentioned earlier it might be a dead end proposition. In fact, both your comments and also feedback I got on Marlinowners lean me towards this conclusion.
The reason I thought about doing it are following:
1) I'm "one gun" kind of person when possible, and my Ruger '44 is an amazingly handy little rifle. It would be really cool to find it's max potential.
2) This is a fairly generic problem. Pistol caliber rifles come in different calibers yet 5 listed restrictions, except possibly rate of twist, apply to all of them. This might be a cool semi-scientific project.
3) Pistol caliber rifles occupy an interesting niche. They are recognized enough to have market and following, yet probably not popular enough to justify extra expense in development and testing for leading powder manufacturers. However, tackling some other challenge for a different caliber, they might've already come up with a powder that, as a side effect, would also be a perfect fit here (that thought lead me to another, to look at more popular rifle cartridges with similar issues, which pointed at AA1680 - that's why I mentioned it in the first post). The solution might be there and just awaits to be found, and if so, the solution would not be a pistol powder.
It appears though that this project has fairly low probability of success. Being practical, I decided to put it on hold until stars align in a perfectly favorable way, i.e. until I will have incidentally collected all necessary components by reloading bench and have nothing better to do.
Leftoverdj: I heard somewhere that AA1680 was one reason why WW680 was discontinued (almost identical). Is there some value to this, or am I totally off?
Unclenick: Special thanks for running these numbers. I never heard of QuickLOAD (I do now. Is it available for download anywhere? Sounds too good to be free.)