Adding non-braced third staff below PianoStaff

[Duncan]

There probably is a simple answer to this, but it has eluded me. 

I am scoring organ music using two staves at the outset, then adding a
third staff later. So I start with a PianoStaff context, then I add
an additional Staff later on when I need it.

Everything looks great, except that the added staff has a curly brace
connecting it to the existing two. This would be appropriate if scoring
piano music requiring three staves, but organ music does not use a brace
between the pedal staff and the manuals.

If I just wanted to eliminate the brace from all three staves, I would
do this, right?
\override PianoStaff.systemStartDelimiter = #'SystemStartBar

How do I state the override when creating the third stave so that it is
not braced to the PianoStaff already existing, but the brace remains on
the upper two staves?

Best wishes,
Duncan