Ways I have witnessed teams stretching the rules:
1. Light cars, one team won 2 pro championships adding lead shot in driver's fire suit between run and scales. One team puts a 35 lb truck alternator on floorboard of driver's compartment, some use heavy body poles for FC's, etc.
2. Large coil of air line between driver's seat insert and seat pan to delay start of retard box. NHRA caught this but no mention because of big sponsor. Electric timers when only pneumatic were allowed.
3..Several additives for alcohol cars that cannot be detected. Don't really know if any actually helped performance.
4. NOS in fire bottles. An alcohol FC won a championship using this technique. Driver would always pull around and away from other cars and top end personnel because the one bottle with NOS would still be hissing. A nitro team tried this but they filled both bottles for a test session, car caught fire, no on board fire system and car burned to ground, driver got out without burns. This was a very big name team.
5. Longer wheel base. I bought a slightly used TF dragster and it wouldn't fit where it was suppose to in trailer, measured wheel base and it was 307". Seen FC's with up to 4" extra.
6. Traction control systems.