Jonathan,
A CLOB column will have the same problem. I think I came across this
problem quite a long time ago so it may be logged as a bug
somewhere.
If you can't find it as a bug on MoS raise it.
If you're licensed for the partition option you could avoid the growth and
the down-time
by adding in a step to recreate the table:
(I haven't tested this with a CLOB/extended varchar2 in place):
Create the table as a hash partitioned table with one partition
Create a non-partitioned clone
Do your delete/insert
truncate the clone
insert append from the partition to the clone
exchange partition with table - online (12c feature).