Hey dude, I just saw this post as I was browsing around and I'm pretty much in the same boat you're in.
From what I understand, I was born in Taiwan and remember where I lived before my family immigrated to the USA. I would have to go to the household registry in Yonghe to get my Taiwanese ID with my ROC passport with overseas Chinese stamp. After which I take my butt over to the "bing yi ke" to get my exit permit or online, but I'd rather go in person 1st time. There's a whole law surrounding that whole 4 month thing, posted here
http://www.nca.gov.tw/EngVer/eng-6.asp, choose option #7
The language used in the English version is weird and I had my parents help look at the Chinese version, and of course that didn't really help much as they aren't lawyers and would be the same as any other 60 yr old American reading legal American documents.. shit gets confusing.
However, I tried to break it down and after conferring with them continuously. What we seemed to have agreed on is that the law says I am able to stay over 4 months but I cannot stay longer than 364 days continuously, twice. If I do, I get conscripted. SO basically, in theory, this is how it would work..
11/15/11: Enter Taiwan with ROC passport w/ Overseas Chinese stamp
11/16/11: Goto Household registry and get ID card so I get to look for work and apply for insurance and all the benefits of being a citizen.
11/17/11: Goto Bing Yi Ke and get exit permit or get online
11/10/11: Leave Taiwan with ROC passport w/ Overseas Chinese stamp
11/10/12: Enter Hong Kong with USA passport
11/11/12: Exit Hong Kong with USA passport
11/11/12: Enter Taiwan with ROC passport w/ Overseas Chinese stamp
11/12/12: Goto Bing Yi Ke and get exit permit again or get again online
11/01/13: Leave Taiwan with ROC passport w/ Overseas Chinese stamp
11/01/13: Enter Shanghai with USA passport
11/02/13: Leave Shanghai with USA passport
11/02/13: Enter Taiwan with ROC passport w/ Overseas Chinese stamp
11/03/13: Goto Bing Yi Ke and get exit permit again or get again online
03/01/14: Leave Taiwan with ROC passport w/ Overseas Chinese stamp and USA Passport (ask them to stamp it)
03/02/14: Enter LAX with USA passport
03/15/14: Enter Taiwan with ROC passport w/ Overseas Chinese stamp
2014: Rinse and repeat until mandatory conscription is abolished, Overseas Chinese stamp now useless and dual citizenship is in full effect. Entering and leaving any country is possible whenever with either passport.
This is just a scenario.. if anyone believes this to be wrong, please correct me!!