Actually DblPAL and DblNTSC *DO* work in ECS.
ECS had two major changes, one of which was the improved blitter (could blit more in one instruction, although the time to blit wasn't much improved since it was still memory bandwidth limited).
The other big change was programmable scanrates. Vertical refresh and horizontal refresh could now be set to arbitrary values within the supported ranges. To support this, higher pixel clock settings were made available, with the limitation that the faster pixel clock modes could only have 2 bitplanes.
Every AGA graphics mode works fine on ECS, as ECS supports exactly the same scanrates as AGA. It's just to do DBLPal/DBLNTSC/etc. you have to set the pixel clock to its highest speed, which limites you to 2 bitplanes/4 colors.
Note that it's actually possible to use a lower pixel clock on a high scanrate screen! For instance you could do DBLPal scanrates, but halve the pixel clock, and have a 320x512 screen in 16 colors on ECS.