Idiot said...
make it to Fremanle and I'm in
1-Park my car in Perth
2-Catch the train from Perth to Mandurah
3-Downwinder to Freo
4-Catch the train from Freo to Perth
I've always wanted to go from rockingham to woodies/freo, but the main problem you will find is that there are large sections of coast essentially closed to the public - ie the CBH terminal, port authority jetties, BP jetties, Hismelt jetties, alcoa jetties, henderson boat yards, etc . Large parts of the beach is private proprety and you can get fined for being there. Particularly if you end up drifing into the jetty zones while they are loading/unloading ammonia or crude oil or something.
I'd also like to go the outside of garden island, but that requires a 10km dash from the tip of garden island back to woodies, i'm also guessing there would be a large wind shadow as you first make the turn. All in all, its a bit treacherous.
You could downwind from basically mandurah to the safety bay pond, and from woodies to freo but the gap in the middle is essentially a no go zone.
I've also been keen to do a run from Meelup to Dawesville. You can drive a 4wd the whole way along the beach too.