Text-indexing UTF-8 bytea, convert_from() immutability, null bytes...

Dear Experts,

I have a table that contains bytea data which sometimes is UTF-8 text.
When it is - and that is indicated by another column - I want to
text-search index it. Something like this:

db=> create index ix on tbl using gin (to_tsvector('english',body)) where is_utf8_text;

(Note my client encoding is UTF-8.)

That doesn't work because to_tsvector doesn't take bytea. So I tried
to_tsvector('english',body::text) ; that almost works, but CRLFs in the
data become \015\012 (i.e. 6 characters) in the text and the tsvector
contains things like '12hello'.

Next I tried to_tsvector('english',convert_from(body::text,'UTF-8')). That
doesn't work because convert_from is not immutable. (This is 9.6; maybe that
has changed.) Is there a good reason for that? Maybe because I might change
the client encoding? As a hack I tried ALTER FUNCTION to make it immutable,
and now I get:

ERROR: invalid byte sequence for encoding "UTF8": 0x00

Hmm, as far as I'm aware 0x00 is fine in UTF-8; what's that mean? But
actually I'd be more than happy to ignore invalid UTF-8 here, since I'm
only using it for text search; there may be some truly invalid UTF-8
in the data. Is there a "permissive" mode for charset conversion?

(That error also suggests that the convert_from is not optimising the
conversion from UTF-8 to UTF-8 to a no-op.)

Anyway: given the problem of creating a text search index over bytea data
that contains UTF-8 text, which may include oddities like null bytes, what
would you do?

Thanks for any suggestions!

Phil.