There's no way to do it. Or, if there is, then it's going to take someone a lot smarter than me to figure it out. However, it might be SLIGHTLY easier to use the iPhone app "itimezone", which costs $2.99. You still have to leave the calendar, but instead of actually changing the timezone of the iPhone, you start up the itimezone app, choose the country and time of your meeting, and it will show you what time to enter in your current timezone...so at least you don't have to do it mentally (and don't have to remember to switch the iPhone back to your current timezone when you're done -- that's the step you save.) As well, you don't have to go through as many menus using itimezone to do this as you do if you're going to settings/mail-contacts-calendars-timezone support/timezone etc.
I'm not affiliated with the people behind itimezone.