Thanks to @steve for sharing this link recently.
It appears that at some point in recent time, Modern Performance is advertising and selling the service of reflashing, or "tuning", the 420A ECU. Most of the information can be found on their website through the link below.
www.modernperformance.com
This is valuable information to this community. More than 20 years of threads have suggested that this is impossible besides via one specific program, which is tough to replicate, and illegal to redistribute.
I'd still lean towards MegaSquirt for specific applications, but it seems to be no longer "impossible" to have your stock computer reflashed or tuned.
I hope to see some input from forum users that have utilized this service for their 420A DSM.
Most search engine queries are a dead end for this topic. Hopefully this will shed some light on the topic to the modern day lurkers.
It appears that at some point in recent time, Modern Performance is advertising and selling the service of reflashing, or "tuning", the 420A ECU. Most of the information can be found on their website through the link below.

Eclipse 420A ECU Performance Flash 1995-1999 Eclipse Non Turbo
Syked ECU Tuning has released a calibrated ECU for the 1995-1999 Mitsubishi Eclipse Non Turbo w/ manual transmission. We have not tested this ecu on an automatic but we are fairly certain this will work fine on the automatic if you want to try it out. If you already have bolt on modifications...
www.modernperformance.comThis is valuable information to this community. More than 20 years of threads have suggested that this is impossible besides via one specific program, which is tough to replicate, and illegal to redistribute.
I'd still lean towards MegaSquirt for specific applications, but it seems to be no longer "impossible" to have your stock computer reflashed or tuned.
I hope to see some input from forum users that have utilized this service for their 420A DSM.
Most search engine queries are a dead end for this topic. Hopefully this will shed some light on the topic to the modern day lurkers.