So in this passage (for piano) I have a pianissimo part in the right hand that follows the left hand. I want the pianist to treat the notes in the right hand 'like distant resonance' (or something like that), although I'm not sure the appropriate place in the score to put the words.
If above the treble clef (as it is in the picture), would that not appear to be describing for both the treble and bass clefs? Should I use an asterisk and explain it separately?
Your picture is correct. If you wanted the dynamic mark to apply to both hands, it should go in between the staves, not above the top staff.
There is nothing wrong with adding text such as "like a distant resonance" if it helps to explain the effect you want.
Actually, there are a couple of other things "wrong" with your picture:
The 8va sign should only extend over the notes, not over the rests. In fact, do you really need the 8va at all here? Don't worry that you might need to increase the vertical spacing between the staves and systems to fit everything in!
The F# and G# on the bottom staff look bad, and the note spacing almost suggests they are played as two consecutive 8th-notes, not together. Either flip the stems so the top note is stem up and the bottom note is stem down, or nudge the G# as far left as you can without colliding with the F# flag. If your "crossed stems" on these notes are meant to indicate the voice leading, you might consider whether anyone will be able to hear that fact - remember music is meant for the ears, not for the eyes!