Phaser LFO idea

[Lincoln Fong]

Forgive me if this has been done but I've always wondered if it would be
possible to build a phaser with an LFO that, rather than the usual GAIN and
DEPTH controls or sometimes just DEPTH, had 2 pots marked TOP OF SWEEP and
BOTTOM OF SWEEP. The advantage being that instruments in different registers
need a different sweep to get a nice even swirl and avoid the part where
nothing much happens.
Put in another way can you make an LFO where you specify the exact height of
output swing, top and bottom using analogue stuff? I'm sure it can be done
but can't picture it. Another thing that springs to mind (that could be
used) is the idea of a slider pot with 2 wipers (homemade?) to avoid
problems of crossing over ranges.

Lincoln