Has anyone successfully put their speakers in storage for an extended period of time?

Yes!

I stored in the original boxes a pair of Spendor SP1 for well over a decade. I now use them in a second system and they are wonderful.  Still sound amazing.  No problems at all.  Wife draped the cartons in lovely fabric.  Topped with glass and left in a spare room looking like a table.
so glad I kept em’.

You should be able to do it. Just be sure the storage facility is "Climate Controlled", not the kind where the roll-up door is outside. I would check them annually for any disintegration of the surrounds.