I found a posting on the MySQL forums from 2005, but nothing more recent than that. Based on that, it's not possible. But a lot can change in 3-4 years.
What I'm looking for is a way to have an index over a view but have the table that is viewed remain unindexed. Indexing hurts the writing process and this table is written to quite frequently (to the point where indexing slows everything to a crawl). However, this lack of an index makes my queries painfully slow.
I don't think MySQL supports materialized views which is what you would need, but it wouldn't help you in this situation anyway. Whether the index is on the view or on the underlying table, it would need to be written and updated at some point during an update of the underlying table, so it would still cause the write speed issues.
Your best bet would probably be to create summary tables that get updated periodically.
Have you considered abstracting your transaction processing data from your analytical processing data so that they can both be specialized to meet their unique requirements?
The basic idea being that you have one version of the data that is regularly modified, this would be the transaction processing side and requires heavy normalization and light indexes so that write operations are fast. A second version of the data is structured for analytical processing and tends to be less normalized and more heavily indexed for fast reporting operations.
Data structured around analytical processing is generally built around the cube methodology of data warehousing, being composed of fact tables that represent the sides of the cube and dimension tables that represent the edges of the cube.
Do you only want one indexed view? It's unlikely that writing to a table with only one index would be that disruptive. Is there no primary key?
If each record is large, you might improve performance by figuring out how to shorten it. Or shorten the length of the index you need.
If this is a write-only table (i.e. you don't need to do updates), it can be deadly in MySQL to start archiving it, or otherwise deleting records (and index keys), requiring the index to start filling (reusing) slots from deleted keys, rather than just appending new index values. Counterintuitive, but you're better off with a larger table in this case.