2 words: adult diapers. That way it's not awkward (keep in mind that the proctors are monitoring your audio through your computer's microphone), and they can't smell you, so who cares? Lol.
On a more serious note, I've been thinking about it, and yes, the mouse-controlled OnVUE whiteboard is a joke, but at least you can use the text editor as well.
I think our best bet on the GMAT Online is to type the math and algebra out as text, in the same way that one might type out a GMAT Club question explanation. Using a mouse to hand-write is just unrealistic. Luckily, I've been typing out math explanations in plain text during my Skype sessions with private students for years (though I will admit that for the hardest questions I will often pivot to a hand-written explanation, matrix or geometry diagram). Thus, I think I'll probably just give up on drawing with the mouse / trackpad. Instead, I'll stick to the keyboard / mental math, and live with the results.