Is it okay to install a sediment filter backward in a reverse osmosis system?
It's hard to say conclusively without knowing what brand or type filter it is, but chances are it's just a fine screen. The concern would be that the cavity intended to collect sediment may not exist on the downstream side. If that's the case it will clog sooner. Otherwise it should serve the function just fine.

Post brand and model information or a photo in your question if you'd like more specific advice.