The pulley should be able to move slime block contraptions, like a piston.

Well I'm not a big fan of this one. Main reason being it would need a smooth animation like pistons have and that might not be possible since it would. Need to have an animated rope or chain aswell to make sense

If its not animated this is easy, jsut use the built in resolver

I would rather have this feature with no animation than not have this feature

because pistons move in a continous manner. thats also how they are able to push stuff up. If they didnt then stuff "pushed" by it would be instantanously moved up 1 block whichwould look bad as it would be too fast and lack any movement.
If one realy wants this they should get create mod