The biggest clues for me were the fashions and fixations with the English language. Interestingly, the word f*ck carries a certain aestheic appeal to Japanese advertisers due to it's symmetrical nature, and as a result appears with quite some frequency and wholly out of context (much to the amusement of the casual profanity spotter).
As for personal emulations, for fear of them being masked by cultural homogenisation, I was informed by a nursing friend of mine that one of the most frequently requested dermatological procedures undergone among Japanese women is a process of skin lightening in order to look more caucasian. However, contrast this with the "wapanese" fashions emerging in the UK and US and it becomes aparent that there are larger scale forces at work. As much as I dislike using the terrible americanisation; "globalisation" is something of a driving force in modern Japanese social change. What else explains the number of yankees 'fans' proudly wearing their caps the world over?
Your point regarding something of a localisation of the globalisation phenomenom is a good one. Once you leave cities and other such hubs of global commerce you will of course see a reduction in 'alien' behaviours and fashions. But, is it fair to expect a blanket-like pattern of acculturation in areas with little contact with outside forces? As for the term 'national identity' I'm not entirely sure such a thing can exist. It implies a very much synchronic state; one that cannot be attained without complete isolation, no technological change and a completely content and static populus. What, for example, is British identity? I'm sure that any definition based on cultural stereotypes (as they so commonly are) will not find widespread acceptance, the Cornish are sure to disagree
Having somewhat derailed the train of thought regarding the nature of this thread I'll rephrase my original summation; boardies would probably be popular in Japan as a result of 'pancultural homogenisation' or of 'an emulation of western fashions and ideals', whichever term stings less.