It is doubtfull that a .454 M1895 will ever be made. One issue is pressure: the .450 Marlin is loaded to ca. 45,000 psi while the .454 is a 65,000 psi loading. Another is action length - it would take some work to get the very short .454 round to function smoothly in an action designed for much longer cartridges.
I'm kinda iffy on the Guide Gun concept myself, but no one can deny that it has been a mega-seller for Marlin - so lots of someones must love the idea.