So, my account can hold a variety of tokens, but my margin requirement is still rUSD. How does that work out?

That’s where your margin balance comes in: it’s the rUSD-denominated amount used to assess your margin coverage. It converts the value of any token into rUSD using oracle prices (using the same ultra fast Stork data feeds that the perps use), then compares the total amount to your margin requirement. The margin balance is the balance figure that you can find in the trading screen.