Okay, after searching for 2 weeks I finally found something that will work for you.
This is the Custom LED Magic Blinker. It takes in a parking lamp signal, turn signal, and hooks up to a single filament bulb. It will do exactly what you want. When the parking lamp is off, turn signal works normally. When the parking lamp is on, and the flasher is activated, it inverts the signal and turns off the light and allows it to blink.
So there you go. $20 apiece and you can use a single-filament bulb as parking and signal.
http://www.customled.com/PRODUCTS/MA...ic_blinker.htm
Also, if you do this you may have the rapid flashing that is indicative of a broken turn signal bulb. You need to get the Libertek adjustable flasher relay and you can set your blink rate manually. Your flasher relay is under the dash, near the steering column somewhere.
http://www.libertek.com/product_assets/prod_fr_p1.gif
These 2 products are used heavily by bikers who swap out their lights all the time. It should work for your car as well since they're all 12V.