Can't use PROGMEM attribute with non - const variables
SPM instruction or not, program memory uses flash memory technology, which requires a fairly elaborate procedure to erase and write new data. There's no compiler that going to let you treat it as ordinary RAM by simply allocating variables there.

You can store data there, but it will always be through library routines that hide the complexity. Going through the library API also emphasizes the fact that this is data that you don't want to be changing very often, because you don't want to prematurely exceed the wear limits on the flash memory.