actually its not anything wrong to do miswaak in public. we don't need to worry about what image the kuffaar are getting of us, unless its really something bad. whatever there is scope for in the shariah, its fine. they (the kuffaar) are not ashamed of the many shameless things they do in public i.e walking around half naked, getting drunk and doing the most disgusting and heinous deeds all over the place, so why must we muslims be ashamed of doing a great sunnah (miswaak) in public? Allahu A'alam.