This is the sort of thing that causes me to give up writing screenplays all of the time.

I have a scene, wherein one person is playing a piano, and that music is overlaid onto a series of shots of things happening at that moment at a another location. What the character is playing becomes the soundtrack for the series of shots if you will...until at the end, we return to the character playing the piano.

Right now, I have just said in the script..

John plays a piece on the piano. A series of shots with his music overlayed onto them.

I then begin to list the short shots that I want..

--we see this.

--We see that.

--we see this and that.

Otherwise, I don't know how else to do it.

That sounds fine, altho I try to avoid using "we see" or "we hear" too often (if ever) -- just b/c it pulls the reader out of the story.

If the music will be playing over something that requires more than a line of description, then I'd use an "official" slug line, i.e. --

EXT. PARK - DAY

Children in the playground chase a pigeon. NANNIES gossip on the bench. One child loses a balloon which soars into the air [blah blah blah etc.]

But if the shots are short descriptions, I'd just use CAPS to indicate it's a camera shot (a commonly-employed cheat, to save room and also the avoid the distraction of slug lines and camera angles) i.e. --

INT. RICK'S PLACE - NIGHT

Sam plays a nostalgic melody on the piano. As the music plays [we see]...

A PARIS STREET where Rick and Ilsa stroll past a cafe...

THE MOON shining down on the River Seine and the couple arm in arm...

THE NAZIS marching up the Champs-Elysee...

THE TRAIN STATION in turmoil as CROWDS rush to get out of the city...

SAM'S HANDS on the piano keys bring us back to the present.

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like that, whether it's a flashback or a dream or a series of shots in the present moment.