Or maybe you should tell them to “kiss my ass.” Having their noses up the asses of Americans, they may not know what an arse is.
That’s interesting what you say about telling them to fake a mainland accent. Some non-waishengren Chinese teachers in Taiwan try to put on a bit of a mainland accent when teaching foreigners. I think their intention is to teach what is considered to be the most standard, and I think their intentions are good. However, a lot of these same teachers do a pretty poor job of immitating mainland Putonghua. When I taught English, I tried to point out non-American alternatives when we came across words like bathroom, cookie, apartment, etc. I never tried to fake a pommy accent, though.
I’ve also noticed plenty of educated Taiwanese who “clean up” their Chinese when they talk to even the most fluent Chinese speaking foreigners. Perhaps some Taiwanese wouldn’t be too irritated with your request. They might just think: “who cares, it’s not my damn language anyway; it’s just my second language.” I often think that both Taiwanese and mainlanders are a bit too obsessed with having one standard. They do it with Mandarin, and I think they want to do the same with English. I think the Taiwanese are worse about it, though. Most Taiwanese I’ve known or taught didn’t care a lick about non-American pronunciation or usage. The PRC, though, has had equally ambivalent relations with most English speaking countries, so they don’t have too much of a preference for one accent over another. A few of the mainlanders I know have a pretty good understanding that English is not like Mandarin in that most native speakers of English don’t think there is one absolute standard for accent or usage (save a few arrogant pricks who can be found in any English speaking country).