I tend to associate increased resolution with objects closer to my eyes which I guess most humans do (lol) which is why I was never a fan of Mode7 scaling where they blew out the pixels when scaling the sprites larger to make them appear as if they were going closer to the viewer.
Anyway I should ask STE 86 if there is any way to make the foreground tiles single pixel? Idk maybe a double wide layer for base color with a single pixel overlay of outline or shadow maybe.
I can't say I'm a fan of mixed resolution either but as far as my Atari research its something I've had to deal with out of necessity, still I think its applicable to the C64.
I think a hierarchy of resolution priority tends to be the logical approach to mixed resolutions which I think is akin to truncation of subdivision in polygonal meshes IE. more resolution in the head than the body, more resolution in the face than the head, and more resolution in the eyes than the ears.
As far as 2D I think character and item sprites benefit greatly from single pixels especially in visual separation and focus to the player. Although Half-Life was 3D I thought the increased texture resolution on charge stations and elevator buttons to that of the world was a positive example of visual focus for players.
Its interesting to discuss but I tend to lean towards what ptoing said about just using double wide pixel throughout instead of adding rastersplits, it just seems hard to add to a game when compared to an art slide.