Yes, you can do this even on progressive, it's known as http pseudostreaming, look it up and you'll find tons of examples for this. Basically you need a piece on your server to recieve the request, in the initial request you'll add via a http header the start point, the server side will automatically move to that offset.
There are better ways now though. In my opinion RTMP sucks, it's very proprietary to Adobe and works over non-standard ports, so many users have issues with it. If you wanna get serious and do it right. Lookup the apache f4f module, adobe http dynamic streaming, and the f4fpackager. Setup a apache server according to Adobe's docs on the apache origin module, use the f4fpackager to chunkify your video/audio into segments. Now point your netstream to the f4m. Not only can you seek crazy fast, your also going over standard http, can use a plain old CDN to cache & server the requests, but you can also support multiple bit-rates for adaptive streaming.