Locking selective text in a field

[Levi Kendall]

Hi all,

  I'm looking for a method to allow selective text to be locked in a
field.  Essentially everything in the field would need to be editable
except specific lines of text.  Or perhaps not even lines, but
specific chunks of characters.  One difficulty is as text is added or
deleted from the field, the sections of text that need to remained
locked will move around.

  Has anyone experimented with something similar or found an
implementation to do this?  Thanks.

-Levi