There are a few things you can try. You could dilute a sulfate shampoo and cleanse with the diluted mixture. IAGirl used to have a blog post on the exact ratio to use, but her blog is down. Basically, with enough dilution, sulfate shampoos become much gentler but can still remove silicones from the hair.

Another option is to use a sulfate free shampoo that contains Cocamidopropyl Betaine. That ingredient will also remove any silicones. Depending on which one you choose, they can be gentle enough to use every time you cleanse. However, some can be more stripping than a sulfate shampoo.

A third possibility is to use a gentle sulfate shampoo or one with low sulfates. Using one with sulfates toward the bottom of the ingredients list is a lot gentler than using one with them at the top, but it will still remove silicones like any sulfate shampoo. CalamityM, for instance, uses a gentle sulfate shampoo every wash, and her curls look fantastic. You could even use the Condition /Wash /Condition method, where you condition the length of your hair and leave it in, then shampoo your roots, and rinse. Then you move to your rinse out conditioner step and proceed with the rest of your routine as you would normally.

I know you said straight cowashing doesn't work for you, but you could alternate it with a sulfate free shampoo when your hair starts getting buildup from silicone use or when your curls are looking droopy.

Lastly, you could try using silicones that don't build up. Amodimethicone, for instance, deposits a single layer, then repels further deposits. It also targets damaged areas, which could be a bonus if you're going to routinely use a flat iron. It isn't water soluble though, so you would need a sulfate free shampoo (low poo) or a sulfate shampoo to remove it. Cyclomethicone (and other Cyclo-cones and Cyclo-siloxanes) aren't water soluble, but according to the CurlChemist, can be cowashed out (or of course low poo'd or sulfate poo'd).
CG since 11/2011
2c /3a F /Mii, low porosity

Autumn Products /Routine:
Clarifying: ACV rinses, Suave Professional Men Deep Clean Peppermint
Low Poo: Renpure Luxurious Argan Oil
Co-washes: VO5 Kiwi Lime Squeeze or Strawberries & Cream
RO: VO5 KLS or S&C, Cure Care
LI (if needed): VO5 S&C, VO5 KLS, Cure Care
Stylers: FSG, L'Oreal Studio Clean gel, Tresemme Flawless Curls Bouncy Curls gel
PT: IAGirl's as needed