Indexable (constant LIKE prefix_keyfield) ?

I was impressed that LIKE operator can be indexed in 7.4 with non-C
locale. But how about the reverse condition?

What I need is to:
SELECT * FROM prefixes WHERE 'literal' LIKE prefix || '%';
or
SELECT * FROM prefixes WHERE 'literal' ~ ('^' || prefix);

Prefix is of type text (variable-length), which may contain only ASCII
chars (database runs under ru_RU.KOI8-R locale). Only the longest prefix
should be taken if more than one matches.

There's no strict definition for "prefixes" yet, and I seek for how to
make it possible to use an index by this query. The ways I see:

1. Sequentially rtrim('literal') and compare it to prefix.
Really bad idea.

2. Use 2 fields: prefix_le and prefix_gt, then
'literal' >= prefix_le AND 'literal' < prefix_gt
(or 'literal' ~>=~ prefix_le AND 'literal' ~<~ prefix_gt, but it
seems there's no need to).

a) supply both fields from outside (I don't like this idea).

b) supply only prefix (=prefix_le), and calculate prefix_gt (using
trigger?) as prefix_le "plus one".

Digging the backend sources, I've found make_greater_string used
to expand indexable LIKE or regexp condition. Can I use it for my
needs somehow? Or have I to write my own in this case?

3. Create some magical index I dunno about :)

4. SELECT * FROM prefixes
WHERE prefix <= 'literal' AND 'literal' LIKE prefix || '%'
ORDER BY prefix DESC LIMIT 1;
Looks like the best way, but I'm not sure this is always correct.

Comments, suggestions, please?

--
Fduch M. Pravking