function indexes, index only scan and sorting

Can someone confirm a suspicion for me ?

I have a moderately sized table (20+ columns, 3MM rows) that tracks "tags".

I have a lower(column) function index that is used simplify case-insensitive lookups.

    CREATE INDEX idx_tag_name_lower ON tag(lower(name));

I have a few complex queries that need to join back to this table (via the `id` primary key) and sort on `lower(name)`.

I'm not selecting `lower(name)`, just using it for an order-by.

The only way I seem to be able to avoid a Sequential Scan and run an index-only scan is with another index -- this one
specifically(and I've run queries against 8 index permutations): 

    CREATE INDEX idx_tag_joins ON tag(id, name_display);

Am I correct in observing that the value of a function index can't be used for sorting ?