Any time I make a RIS I always use yeast that has been harvested through 2-3 generations.....makes a huge difference.
Also, another strategy would be to leave it as is....... 1.048 is actually right in the wheel house of where a lot of the big RIS's are at (Dark Lord, Toppling Goliath Stouts, etc.) Often these beers are barrel aged and you want high FG to blend with the way the barrel can sort of "thin" the beer out.
I would worry at some point, you are either going to ruin what you have by dumping too much yeast and starter cultures in it, or, you might "succeed" with something like champagne yeast and end up with a 1.010-1.015 beer that is really thin and harsh.
So, one alternative would be to add a good dose of charred oak cubes and let it sit for 2-3 months. Could also add coffee beans for a short time (12-18 hours is all) before moving to a serving keg off the beans.... those things actually play very well with a RIS in the 1.04 range.
Just a thought.