Query about tone and stress in Mandarin foreign transcriptions/loanwords
I don't agree that "Mainland Mandarin" is a stress-timed language. Where did you get the "general agreement"? In addition, I think that Mandarin is Mandarin. The difference between "Mainland Mandarin" and "Taiwan Mandarin" is smaller than British English and US English.

Back to your question. Even the words/names are translated from other stress-timed language, one reads them character by character in a steady pace (so called syllable-timed). 布拉德・皮特 reads as 布/拉/德・/皮/特, not 布/拉德・/皮特 or something else. But since many educated Chinese know English very well, they may mix English words in directly when they speak. Nowadays, when one use a standard way to say, for example, 布拉德・皮特, it sounds OK but a bit out of fashion. So people, especially young people prefer to use Brad Pitt instead of 布拉德・皮特. Here the words are already English, not Mandarin or Chinese any more. 朋友 reads as 朋/友. Both characters take the same time to pronounce.

Don't trust websites, such as this. In the comment you already see it's full of mistakes.