legitimacy of using PG_TRY , PG_CATCH , PG_END_TRY in C function

I have a C function (a trigger function) which uses the PG_TRY()
construct to handle certain ERROR conditions.
One example is where invoked as INSTEAD OF INSERT into a view. It
PG_TRYs INSERT into the real base table,
but this table may not yet exist (it is a partitioned child of an
inheritance parent).
If the error is ERRCODE_UNDEFINED_TABLE, then the CATCH issues
FlushErrorState() and returns to caller who CREATes the table and
re-issues the insert.
All works perfectly (on every release of 9.x).

But in a different part of the same trigger function, there is a
PG_TRY , PG_CATCH of a CREATE INDEX,
which sometimes hits error "relation already exists" (i.e. index with
same name indexing a different table).
The CATCH issues FlushErrorState() and returns to caller who ignores the
error.
All works but not perfectly -- at COMMIT, resource_owner issues
relcache reference leak messages about relation scans not closed
and also about snapshot still active. I guess that the CREATE has
switched resource_owner and pushed a snapshot, but I did not
debug in detail.

Those are just examples, but my question is whether use of this
construct is legal in a C function
(always? sometimes e,g only for DML but not DDL? never?),
and if not, then how can one legally catch ERRORs like this in a C
function?
(Yes I realise I could write selects of pg_class to avoid these two
particular cases rather than the try-and-fail approach
but I would like to understand the general rules where it may be much
harder to try avoidance)

Cheers, John