I'd like to implement this small feature --- making table's auxiliary structures store their data to the target table's tablespace by default. I've done a thorough search over the mailing list and there is nothing relevant. Well I may miss some corners :-)
I had a similar problem in writing the range_partitioning extension: CREATE TABLE y (LIKE x INCLUDING ALL) didn't set the tablespace of y to match x.
I don't have a solution, I'm just indicating a similar need in userland.