Managed to get DblPAL working in BetterWB. The issue is when you save, all the icons turn pink and then the entire MiST eventually crashes with a Red Screen of Death. However if you hit the physical reset switch at this point, it boots normally.
Something still looks "off" though. Not sure that the aspect ration of the GlowIcons is right, and the text looks more "messy" than in regular PAL HiRes.
Still probably going to stick with my PAL Overscan 16-color WB. It's faster and saves a bunch of Chip RAM.