"What about adhoc queries using UPDATETEXT/WRITETEXT?"

I would never allow such a thing, unless it was in a custom SP that updated the audit table in later statement.  The same is true if you update a field in the original table:  I would never allow it outside a custom SP.  Therefore, I would not worry about the trigger-firing issue.  But it is good that you pointed out this undocumented (in BOL, AFAIK) problem.

I would guess that your need to write/update text blobs line by line is very unusual.  I guess that >90% of databases will not encounter this issue.  But if you need it, you need it!

Good luck,

Rich